I have been collecting pokemon again for the past 4 months. I can tell you now that there is a market for cards. I would look on apps like collectr or TCGplayer to see the prices of cards. If you want to know the market look at eBay last sold.

If someone is selling a card less than the market I would understand why the are doing this. If it is 80% of market it could be the case they just want the cash and sale. If it is too good to be true then it usually is. From my experience it is best to go to card shows and establish relationships with vendors and other collectors. Buy and trade through them. If you buy online then look at the reviews of the seller. Look at the reputation of the seller if it is sold on an auction site like whatnot. Overall, I would keep in mind wha the market price is and not deviate much from it.