Mr. Wonderful,
This exact same scenario happened to me last Friday on a little Versa-Vise here in Dallas. I saw the post on OfferUp and it was just about 5 mins from my office...of course I saw the ad shortly after I had pulled into my driveway some 15 miles away. I reached out to the guy and said, "hey, I'll take it, but I just got home and don't want to fight traffic, can I pick it up on tomorrow or Monday?" And I went on to tell him that I'd be happy to pay him 50% now, to hold it, through PayPal, Venmo, or the OfferUp app. He says, "Sure man! No problem, I get traffic. What time you want to come by Monday?" I respond that I'm pretty open, just let me know what time works for him and I'll make it happen. Not 10 minutes later I get a notification saying that the vise had sold. I reached out to him asking if he canceled the auction for me and he replies that someone offered him full price and already picked it up.
It was frustrating, but it happens. I suppose, like Mr. Wonderful, that I just wish the sellers would be more forthright from the start. Either say you're willing to wait and or say, "sorry, first come, first serve" and that's that (and totally reasonable, might I add).