What does it mean by selling 50 Venmo

What does it all mean by selling this or that how does it benefit me

It means the player has $50 in his account that he wants to sell.
You would send him $50 by venmo and then he will send the funds to your account.

But what’s the point of that when I could just buy my own bitcoin

A lot of people don’t want to use crypto.

Got it thanks