YES, although this occurrence is not specific to OfferUp. i've had it happen when selling items on craigslist or fb marketplace in the past. someone messages me about an item, we agree on a price, and they show up and say, "i only brought this much money" or "i thought you'd take $xx". sometimes they put a guilt trip on you because "i came all this way to buy the item".
this actually happened to me last week, someone shorted me a couple bucks on a sale. i know i should have told him to come back with the full agreed-upon price, but i didn't want to get huffy over like $2. i did tell him it was a rude move though, and not the way to do business. actually, i'm kind of getting upset about it in retrospect, haha.
the problem i see, and i have had a lot of positive selling experiences here and elsewhere, is just that a certain percentage of buyers don't really understand basic business rules & etiquette. pretty sad.
sure - except that many people agree on a price, given that the item is in a certain expected condition, before meeting. i'd say that's the 2018 version of a handshake agreement.
what do you think would happen if you went to an auto dealership, neotiated a price with the salesperson, shook on it, then then when it came time to sign the paperwork you said you wanted to pay a few thousand less than the agreed-upon price? that's a really bad look.