As we all know, Yu-Gi-Oh can be quite an expensive game to get into. That's why it's imperative to spend your money wisely and plan accordingly! Today we'll be covering the Product Blacklist, which comprises of stuff to avoid for both new and returning players wanting to play in a semi-competitive manner. We'll also be giving some recommendations afterward.
As a general rule of thumb though, avoid loose packs and booster boxes. The former is an easy way to lose your money fast, while the latter is often more so for collectors or ones seeking the thrill of the hunt.
Disclaimer: The products that are on the blacklist are actually fine for casual play or other purposes, this guide merely serves to raise awareness and to make sure you spend your money wisely in this not-so-cheap hobby.
Credit to Yugipedia for some of the images used, feel free to check them out here! Special thanks to the r/yugioh Discord server as well for helping create this article.

Outdated for Starters

First things first, for the Product Blacklist; it is NOT a good idea to get Starter Decks at all for the most part. Especially these outdated ones like Yuya who are still being sold on shelves for no good reason. This also goes towards products that are older than 2017 at this point. It's not a good way to start your footing in this game as the strategies and staples here are lackluster.
But then clearly the modern ones are up to par, right? In all honesty, no. They're some of the worst products of the decade, Harpie's Feather Duster being the only consolation prize. Casual play sure, but even then that's a stretch. Your $9.99 is better spent elsewhere if you want to start getting into the competitive circles. Stay away from these at all costs.
The next best thing comes to mind is then Structure Decks right? Well yes and no. While it is true that some of the Structure Decks are definitely solid and are a nice 3x pickup, it is highly advisable to investigate further before gunning for them. Unfortunately, there's a lot of stinkers out there too.

Structurally Unsound

Structure Decks are usually fine, but these ones are downright predatory and terrible. it is worth noting that they used to be good, but that was years ago and they did not age well. This applies to most, if not all of the ones I'll be talking about. Just because they were good in 2016 doesn't mean they're good purchases in 2021. The Below 2017 rule applies as well, so no going out there purchasing Geargias or anything wild.
Blue Eyes and HERO are the biggest examples of this. They keep victimizing newer players. Dark World got a recent reprint too so steer clear from that. The Cyberse Trio did have some good cards, but it's impossible to build a cohesive deck out of them. The Yugi / Kaiba ones aren't good either. If you wanted to play ABC, go pick up a core for cheap. While strategies like HERO are still technically playable, these aren't the products to buy if you want to get started.

Egregious eBay Decks

eBay itself isn't the worst place to get cards, but extreme caution must be taken into account when purchasing there. The site is infamous for hosting these "ready to play / tournament ready" cores or decks. They are often overpriced, misleading, and incomplete. It is often just 3x copies of cheap archetypal cards that don't actually reflect what a competent version of the deck should be.
For example, this supposed Salamangreat list is dubious and has a lot of cards that are not used in the deck at all and then some. Stashes and lots, in general, are also never worth it 9 times out of 10 and those also float around in eBay on the occasion.
Consider these two some of the worst offenders on the Product Blacklist.

Nostalgia & Miscellaneous

The nostalgia-focused products that cover the Duel Monster will also be included in the blacklist for reasons that are easy to identify. An example is Yugi's Legendary Decks as shown above. They provide little to no competitive value to the scene. Once again, these products are mostly just for collectors.
On the other hand, Legendary Duelist sets may have big ticket cards on the occasion such as Magicians' Souls or Baronne De Fleur, but that doesn't mean it's a good idea to chase boxes or packs in hopes of pulling them. The same can be said about Brothers of Legend. It has a wide variety of good cards, but money card pull rates are as abysmal as ever and it's quite expensive to get.
Sets such as Hidden Arsenal and Dragons of Legend aren't exactly worthwhile too. Skip them too and you'll be happier overall.
The Speed Duel Boxes are good but strictly for Speed Duels, but don't purchase them and expect you can build off them in the actual TCG. Speed's differences with the main game are almost night and day.

Your Mileage May Vary

This section will be talking about a "good" product that has a caveat to consider. Duel Devastator is an incredible purchase that also helps jumpstart your deck with Hand Traps, staples, and all other important techs. However, your mileage will vary depending on the price you'll find DUDE at.
Ideally, you'd go for below MSRP but somewhere around the $30-35 range isn't the worst. Ash Blossom currently commands $20, Solemn Strike and Ghost Belle are also $5 a pop. Supporting cards such as D.D. Crow, Veiler, Gameciel, Dinowrestler Pankratops, Called by the Grave, Cosmic Cyclone, Artifact Lancea, and Super Polymerization help soften the blow and make the product worth it to a decent extent. All of the mentioned cards are highly playable and often see meta play either in the Main or Side.

Recommendations

Now we've talked about stuff not to get, what's next? Let's take a look at some things to do that'll lead you in the right direction. Picking up singles over sealed products is usually the best advice, but here are some other good things to keep in mind.
For your shopping needs, NA has TCGPlayer as their main store! Make sure to stick with Verified Sellers, the ones with an orange icon. EU has Cardmarket, and it's a good idea to stick with Professional/Powersellers with a little person icon near their name.
AU/NZ has some good shops such as Cherry Collectibles, Nakijo, and Chaotic Factory! Canada has a wide variety of options like Cardbrawlers, Dollys, and Carta Magica.
If you're not sure which deck is the best fit for you to pick up, feel free to check out JebusMcAzn's wonderful Competitive Budget Masterpost list here! This list gets updated every so often when a new banlist arrives, but as of writing this article, it's the latest version. Here you'll get to see a wide variety of competent decks that can perform decently in a competitive environment.
Slowly upgrading your deck is a good call as you don't have to spend ludicrous amounts of money from the get-go. It's often a common misconception that you have to bust out obscene amounts of cash in order to keep up with the game. This is a poor claim that is both false and blown out of proportion. Owning staple cards also reduces the cost of building as they can be used in almost any strategy.
Just take your time and plan your budget accordingly! Hand Traps, powerful Extra Deck options, and important generic cards are notable things to get after you get the core of your deck rolling. Cheap techs can also fulfill similar roles in some cases.
Lastly, getting basic accessories such as Sleeves, Dice, and Deck Boxes also go a long way in terms of playing the game. Ultra Pro, Dragonshield, KMC, UG Katana, and Ultra Pro Eclipse sleeves are some decent options that all have their pros and cons.

Conclusion

And that wraps up our quick guide on what and what not to buy for new and returning players! I hope this can help players make informed decisions with their purchases. Feel free to link this resource to aspiring duelists out there!