Does anybody know about collecting sports cards, like authenticity, and value?

I have a bunch of sports cards. They were a kind of a garage sale deal. There is a bunch. I don’t know anything about them. I thought it would be kind of easy to look up if they are worth anything, but I was wrong. I am confused, because some of them seemed like pretty big names, but the listings I see on ebay are all over the place with the prices.
Can anybody help me? I was going to find a place to see about selling them, but I don’t trust those shops much. I feel like they are going to lie to me so they can get something of value for next to nothing. If I could find just one valuable card in the bunch it would be worth the cost of buying them.