Yep, there is Kijiji www.kijiji.com.tw
Personal favorite is Yahoo Kimo bid: www.tw.bid.yahoo.com
To give away, it si very simple: try the elderly people who push their carts along the road. If not, Buddist Charity Tzu Chi will take large and small stuff off your hands gratefully as a donation.
The sign thing is not as common, would freak people out me thinks. Too embarrassing, moreover.
International schools hold garage sales like twice a year. Great bargains from people leaving the Island. I'd say best chance is to get in touch with other foreign moms in the vicinity for play dates and clothes exchange.
Locals do not like to buy 2nd hand that so much, but recently there are a couple of store chains that focus on that market.
Books at Mollie. They have one in Kaohsiung, at least, I have bought from them. www.mollie.com.tw