Almost every merchant that buys on exchanges are traders who use it as a means to acquire coins to place their trades. I've come across those who even urge you to release fast so that they would use it to follow up the markets. The buy price is similar to sell price on exchanges, so I don't see profits there.

I don't think merchants make money on P2P directly on exchanges, there may be those that buy from there and sell offline at a higher price to crypto illiterates or those that buy offline at lower prices and sell at normal price at exchanges,

The only ones who make money strictly on exchanges P2P make it illegally, maybe those merchants who scam people by paying lesser than designated amounts.