Overselling means offering a prospect more than they actually need or have ever asked for.
It usually happens when a seller is so eager to sell something that they start to overhype their product or service. This might also include exaggerating some qualities of the product or service.
This term can also be applied to describe excessive pitching, especially if a prospect is already about to buy. In this case, trying to oversell can eventually make the prospect change their mind and prevent the deal from closing.