Welcome to the hobby and the forum! You’re definitely in the right place. Before I answer the specifics of your question, I want to echo @mrbubbles: just avoid Mercari. There’s effectively no buyer protection on middleman mercari purchases, and many of the issues you rightfully bring up will be minimized just by purchasing from a more reputable outlet.
However, here are some answers to your questions anyways:
Personally, I look at the card condition, price, and the seller. First off, what does the card look like in the pictures? Generally you can figure out if it’s played, excellent, or near mint from the photos. If it’s ungraded and expensive, you want to be skeptical of why it hasn’t been graded. Yes, some people don’t grade cards. However, tons of people do and they’re trawling mercari all day. These days, if it’s available for sale for more than a few hours, it’s probably either (a) priced pretty highly, or (b) there’s something fishy about it. Compare the price to what you find on eBay or other auction sites for a relatively equivalent graded copy, probably lower than what you would think it would grade. Chances are it’ll be more expensive and that’s why it hasn’t sold. However, this is probably a good sign, because the scammers price items to sell and real people often don’t. Next, I look at the seller. Do they have good reviews? Are they blacklisted on any major middleman sites (red flag if yes)? Are all the photo backgrounds similar (red flag if no)? If all that checks out, it’s probably a legit card.
You can never be sure. You should learn the key hallmarks of many fakes (no or incorrect holo pattern, incorrect font, bad color saturation, etc). Especially on sites you need a middleman for, you can’t be certain they’ve posted pictures of a real card and will then send you a fake. Do a reverse image search or check the seller’s other items to see if anything looks suspicious.
It’s more risky to purchase ungraded high value cards. The market is so optimized now that if something hasn’t been graded, there’s likely a reason why. Either it’s overpriced (best case scenario), the seller is fishy, or you’ve encountered the extremely rare and highly unlikely case where you’ve found a steal of a deal before anyone else. When you’re new to the hobby, the best thing is to minimize your risk. Stick to trusted sites and graded cards, while you will pay more, you’ll have no risk of getting scammed.
), I will definately be skipping this listing.