The Shop a Player screen is where general managers can shop a player for potential trades:

You can specify the type of offer you want in return - a single player, a multi-player package, or a prospect package. Depending on the type of offer, you may also be able to add specifics about the kind of player you want to receive.

The process of making a trade can be pretty involved. Please see Trading for an overview of the entire process of conducting trades in OOTP. See here for information specifically about the Shop a Player page.