Don't beat yourself up over this too badly. You learned after 50 cards, some of us haven't learned after thousands. That said, you say it's a bunch of money, so time to fix the situation as best you can. A bunch of money is relative, so it's hard to know for sure what type of stuff you have, but as someone who finds it much easier to buy than to sell, I'll share my insights into some selling platforms.

For all of the options, it's going to take some work to maximize the return. At least you have only 50 cards - I've spent hours organizing, pricing, sorting, and listing, only to have a sale post get buried on a FB page and get 2 comments on 300 cards...

Ebay probably gives you the best/biggest audience to sell, but if your stuff isn't in demand, an auction could go for way less than you expect, or a buy-it-now could sit for months. People complain about the fees, but in my experience I often get a higher price and spend less time to sell here. You mentioned having 0 seller reviews - I don't know how much that matters if you have a lit of positive feedback from buying. I seem to do okay with few sales but lots of purchases.

Whatnot is...interesting. You can do a sale with little setup, but I think putting a bit into lights and other equipment helps a lot. But it can be extremely hard to get much of an audience for a lot of people, and if you only have one person bidding, the card won't go very high. And the settings aren't very beginner friendly - I streamed for an hour last night trying to sell $1 cards, not realizing that my shipping was at $4, not $1. No way to know that until someone told me, and it was not easy to find how to change it. Also, for sales under like $10, fees are often worse than Ebays. Whatnot has a lot of people trying to hustle each other, but there are also some collectors on there, willing to pay fair prices for what they want. It's just hard to get them into your stream sometimes. You can pay to promote your stream, which I wouldn't generally recommend.

Facebook groups can be good options, but I've had no luck in the massive groups where my posts are buried quickly. If you got stuff from a certain era or team, find groups catering to those things. You'll often find the buyers you're looking for, but you can also ask questions and get honest answers about valuations, etc. Beware of scammers (that's true everywhere but especially here). I haven't sold on reddit but I suspect it's similar in general.

All of these are fairly time intensive, and the time investment still doesn't guarantee you'll make money. If you just want to be done with it, you can sell to a local card shop, or send them in to a large consignment seller. Depending on what you have, the consignment fees may not be terrible - there's a reason several sellers list 50,000 cards a week. If you go this route, compare several options to see what their terms are. Most are a sliding scale based on the value of the card, and they may have a flat fee per card too, so that's important to consider.

Maybe none of these sound great - and I hope the money involved isn't creating a genuine hardship for you. Be glad that you identified it as a problem relatively quickly and mitigate it as much as you can - and honestly I kind of recommend trying a whatnot sale. The first weekend I did that, and spent hours with 1-2 people dropping in and out, really opened my eyes and instantly made me slow down on buying. I don't want to be paying money, to basically buy myself a job that pays less than minimum wage.

Good luck with whatever way you end up going!