I started selling on Mercari an I have I say I absolutely love the options they have for sellers. You can offer to do your own shipping which I do quite often as it doesn't cost $5 to ship a 3 ounce item, I simply go to pirateship.com and buy a label, ship it out and send the tracking number to the buyers. Yes, you still get some lowball offers on there but not even close to the crap people try to pull on here. They also have a counteroffer option when someone sends you an offer and I have made quite a few sales with that, selling something for 8 or 9 and they send me an offer for 6, I counter with 7 and bam, instant sale and both parties are happy with the price.
Offerup needs to step up their game or they are going to fall behind all these other selling venues, sure its great for people wanting to meet locally but I prefer the sell and ship model myself, I dont have to to run around and hope someone shows up at the agreed meeting spot - been burnt fom that a few times so I refuse to meet anyone anymore, they can come pick it up 2 mins walking distance from my house or have it mailed to them.
Ebay lets you set your shipping prices as well and has the option to set offers and reject offers lower than the price you set, offerup would be wise to implement some of these things, especially new shipping options! I refuse to even list books or heavy items on here because I am not charging people the insane shipping prices they have in place. Small items are being charged way too much to ship as well but I list those and have sold many, but again... anything under 15 ounces should NOT cost $8 to mail and items under 4 ounces cost less than $3 most the time.