They knew it was stolen. They knew the guy found it and didn't return it to the owner (and did not go through the right avenues to make a real attempt to) and therefore it was not his to sell. Doesn't matter if it was a real iPhone or not. They could have just liked the really pretty fake and thought it was a fake. Doesn't make it any less of buying stolen goods.
The only way they could claim they didn't know it was stolen is if the guy claimed he rightfully owned it (that he didn't find it, that he legally bought it or went through all the right avenues to make sure it was legally his) or that for some reason he represented Apple computers and was given the power to sell the iphone from Apple and they wanted to sell this iphone to them.
In which case, I doubt they'd pay 5k cause he wouldn't rightfully own a prototype iphone (the guy who lost it didn't "own" it either) or do you really think they'd pay 5k for a random iPhone fake that they actually believed/knew was a fake? That's really stretching it. And I think anyone with common sense would know Apple wouldn't be trying to sell them the prototype iPhone (or that it at least wouldn't just involve one random person claiming he represented them).
I honestly think some one had a good theory when they said the guy may not have even tried to return the phone but Gizmodo told him a cover story (including calling Apple Customer service to have a paper trail) so that they could pretend it was legal. I mean who would wait around at a bar for an hour for the owner to return but not answer the phone if it rang or at least mention to the barkeep (Hey, I found this phone, I'm going to hold on to it but if the owner calls, I have got it right here). If he truly was trying to find the owner, he would have at least told the bar that he had the phone, maybe left a number with them for the phone's owner to call him if he came back looking for the phone. I mean I can understand the arguement of not trusting the barkeep to not steal the phone, but people who want to return something who don't trust a retail person, will tell the retail person they have the item and usually leave contact info. I know, I work retail and see it all the time.