I did one OfferUp transaction for a Paramount that was posted in the "are you looking for" thread. It was time-consuming, the seller was clueless but nice and needed the money (her price was too low, I offered a little more to salve my conscience and to give her incentive). I had to walk her through every step as well as make arrangements with a local bike shop to act as an intermediary. Worse, she could only do Western Union which added new levels of complexity and risk.
Unlike a prior transaction for another Paramount via Craigslist that went pretty smoothly, I had heartburn at every stage of this one. Still, it worked out well in the end and I wound up with a beautiful bike in excellent condition for a very reasonable price. Would I go through that again? Absolutely not. I'm pretty done with OfferUp.