Tough to measure there. A profession, to my mind, is something someone does as your main source of income external to your own needs. As such, I think it would be hard to say when farmers or hunters became "professionals" as opposed to just selling/trading surplus.

Did the building contractors only build houses for other people as their main employment?

Due to the nature of the work, I'm guessing trading sex for goods and services has been around since the dawn of human civiliazation, but men just didn't want to make ledgers noting this until 2400 BCE.

"Hey honey, didn't we used to have two brooms? Barbara down the street has a broom that looks just like one of ours."

"I don't know, darling. I traded Bob our old broom for a beaver pelt and melons months ago."

"A beaver pelt and melons, huh?"