1. Let's say someone offers you less than you're asking for something, and you accept. Before you've set up a pick-up time, someone else offers full-price. Do you tell Buyer #1 that you're no longer able to accept their offer, or do you feel obligated to take the lower price?
2. Let's say you get multiple offers on something, and you accept one. How long do you give that person to set up pick-up before offering to the next person?
While I'm sure it's done all the time, I would feel uncomfortable going back on an agreement. If you agreed to sell it to someone, I'd go through with it.
As to the other, I'd give the person a timeframe to get it, and if they didn't get it within that time, move on.
I tried to buy a double stroller off of cl and offer was accepted. I had M when I was supposed to pick it up. I emailed to say I would be late but still wanted it.