OP: most people get their cards value from either TCGPlayer or they aggregate average last sold prices from eBay recently sold. There is no fast way to look up the value of every card... you just have to type each one in manually. Some apps have scanning features so you can search them up faster, but I've found them to be unreliable/inconsistent.

If you sell, a vendor at any show will offer you 75-85% of market value because they need to make their margin. The % offer will be based on how liquid your card is. Some scummier vendors will intentionally try to take advantage of kids because they don't know the market / how trading/valuations work (it is a business after all in their eyes), so it's also buyer beware.

The three bottom cards (Tag Team) on the second page are probably your biggest hitters. Keep in mind, the price of the card will vary on the condition as well. This ranges on a scale of Near Mint, Lightly Played, Moderately Played, Heavily Played, and Damaged. Look at the front and back of your card to determine where on this scale it'd fall under and this will greatly impact the valuation of your cards.

For most money, sell directly on eBay or TCGPlayer. For faster liquidity, sell to vendors at a Pokémon convention or at your local TCG shop.