Did I sell my black lots for a good cost?

I had some old magic cards i took to a card store and I sold a black lotus card for 50 dollars. it was the only one of my commons he wanted to buy. Then I was bragging to my friend about how i ripped this card shop off (seriously i can buy like 300 mana with what he paid for it) and he said that I could have sold it for thousands. He likes to make fun of my knowledge of card pricing but come on for a peice of cardboard no one will pay thousands for that card! So did I rip him off badly or what?