Hagrid didn't trade Fluffy for the dragon egg. He won it in a rigged game of cards.
Hagrid was accosted by the supposed dragon dealer in the pub and persuaded to play cards, with the egg as the wager. It's not clear what his stake was, but he doesn't mention putting his own pet up as the bid, only wagering on the egg (possibly against cash from earlier hands).
‘Where did you get it, Hagrid?’ said Ron, crouching over the fire to get a closer look at the egg. ‘It must’ve cost you a fortune.’
‘Won it,’ said Hagrid. ‘Las’ night. I was down in the village havin’ a few drinks an’ got into a game o’ cards with a stranger. Think he was quite glad ter get rid of it, ter be honest.’
Harry Potter and the Philosopher's Stone
Note that Quirrell/Voldemort expertly pumped him for information and bought him multiple rounds of drinks, so there may also have been multiple hands of cards prior to the big wager. Hagrid honestly doesn't remember too well, because of the aforementioned free drinks.
I can’ remember too well, ’cause he kept buyin’ me drinks … Let’s see … yeah, then he said he had the dragon egg an’ we could play cards fer it if I wanted … but he had ter be sure I could handle it, he didn’ want it ter go ter any old home … So I told him, after Fluffy, a dragon would be easy …’
Note that in the film Hargrid doesn't say that Fluffy was 'in trade', but rather that the dealer and he are both in the trade (e.g. the animal breeding trade).
Harry Potter and the Philosopher's Stone - Original Screenplay