I started selling a bunch of stuff on Mercari last year, which I believe is owned by eBay, but have gotten similar DMs. I'd say 1 out of every 10 messages usually leads to an actual sale. I really don't understand why people ask, because Mercari has the option to offer, and most people offer $5-10 less than what I'm selling for, so I typically accept the offer anyway.
The question that gets under my skin the most is when people ask me to hold an item I've just listed for 5-10 days until their next paycheck comes in. I always tell them I'll do it for a down payment, and so far nobody has followed up on that offer!