What are the most used online second hand stores in Taiwan where I can not only buy things but also sell things?
I know Facebook Marketplace, but I’ve had quite some unpleasant experiences with Facebook, so I try to avoid Facebook as much as possible…
What sort(s) of “things” are you referring to?
I’m uncertain if it will be of help to you, but forumosa has a “Forumosafieds” forum to buy and sell. You can access it here:
Guy
I’d honestly just use Facebook Marketplace and meet up in person rather than doing COD, but you can also buy and sell second-hand stuff on Shopee (for buying there’s a filter for second hand, for selling would need to create a seller account, which isn’t too much hassle).
I guess there’s Yahoo and Ruten too, but I’ve never really bothered with those.
I will be selling mostly electronics, bikes, some household stuff, clothes and shoes. I’m moving and can’t take everything with me.
Alright, I’ll take a look at “Formulosafieds”.
I’ll take a look at those pages. Thanks!
If you’re selling branded clothing in good shape, then the growing number of 2nd Street shops in Taiwan may be able to help you out.
Please don’t bring dirty trash to them as this wrecks things for the rest of us. 
Guy
I’ve encountered people who insist on COD for some reason. I’m not sure why but they write as though they’re in prison or something.
I sold a camera to someone from Indonesia and she seems unable to even search for a lens on facebook marketplace. Either they’re dumb, or they are playing dumb.
For physical place to buy/sell second hand stuff, they opened a Hard Off in Taoyuan.
Another Japanese player in Taiwan! 
Guy
Not a shop, but just use Shoppe. Search and tick the box for second hand. A lot of actual second hand stores are not a good bargain, but you can often find ordinary people selling things. FB Marketplace is practically anonymous because many people use fake names, but you can check a seller’s history on Shoppe. The most annoying things is that many people sell things off Shoppe but leave the listings up.
I guess that’s why some people keep insisting on COD, probably something fishy going on.
Same with buyers acting like literal infants to where they don’t even know how to receive stuff through 7-11 or whatever.
Regardless of what forum you use to sell your stuff, you should remember that if you want to sell, price it to sell. Too many people think their stuff is worth a lot more than it actually is worth. Good luck.