Hey,
Just wanted to provide a rundown of places in Taipei to buy and sell used books, since it's a question that gets asked a lot and definitely deserves a solid answer available online. You might think that a city like Taipei wouldn't have these options - we're not a backpacker haven like Bangkok (thank the gods!) nor are we an expat hell like Seoul (again, I thank thee, O Heavenly ones). Options do exist, however, and these are the ones I've found so far:
Whose BooksUpdate: this storefront is gone. Whose Books' main store has moved to Gongguan. The sign is visible on Roosevelt Road right where Xinsheng S. Road terminates and the entrance is in the back (enter the lane and turn in to get to the entrance at the back).
Another branch can be found in Shilin - MRT Shilin, main exit, but after you exit make a U-turn to the right to backtrack down under the elevated track and it's at the back of that public square area. English books are upstairs.
Both stores buy used books (but don't give much)
Best selection of used books in Taipei, and seems to be getting bigger. They've got something for everyone - nonfiction, sci fi, old guidebooks, cheesy chick lit and romance novels, serious fiction for serious people, Booker Prize winners, backpacker fare, self-help, technical manuals, whatever. You can sit on ledges or on the floor and there are 3 tables in the back. Coffee and water available. Will provide a "VIP Card" for discounts, and will buy books but not at a competitive price (if your aim is to get rid of old books and make space for new ones, not to make money, it's a good deal). With the tables, good selection and windows in the back, it's a great place to spend a rainy Saturday, finishing with some wine and a smoked salmon sandwich at the cafe down the street.
Mollie's Used Books I
Taipower Building MRT - take the southernmost exit on the west side of Roosevelt Rd and walk down past the actual Taipower Building. Turn right in the lane next to the building painted bright yellow (used to be Asto Gelato if you know the area, now it's a Bossini). Walk down past Karma and the Buddhist Library and turn left in Alley 10. It's down a little ways.
Of the three Mollie's, this one has the best selection of English books, but that's not saying much. There's a small selection mostly of self-help and business "How to Re-Engineer Your Blue-Sky Deliverable Envisioneering" wankology and some cheap sci-fi paperbacks, among a few books actually worth reading (not that I'm dissing "The Return of Xargax" or anything...oh wait, yes I am). But they make OK coffee for 50 kuai, have used CDs that are sometimes good and sometimes horrific (Chumbawamba?). Downstairs there are a lot of cheap kid's books in Chinese, good that's where your Chinese reading level is. Finally, downstairs there's a decently eclectic selection of old guidebooks. They say they buy books but they wouldn't take ours. Strange, as our books are better than what they've got.
Mollie's II
South side of Heping East Road between Shida and Xinsheng S. Road. I don't remember exactly where but it's a basement entrance and very easy to miss, so look carefully. It's near that Chinese restaurant that looks Italian, which is next to an actual Italian restaurant.
This one has a few tables and benches, and they don't mind if you use the floor. They also have 2 cats and seem to provide coffee. One small nook on the far righthand side of the store has English, German and some French books, and there are a few used CDs. This is a fun place to grab random stuff and read away a day, but don't come expecting to find anything. I've never tried to unload old books here.
Mollie's III
Somewhere in the crazy lanes of Gongguan Night Market - good luck. It's near a Vietnamese restaurant.
The only English selection seems to consist of old textbooks and manuals, but they have a pretty good used CD collection. Doesn't matter as nobody can ever find the place.
Grandma Nitti's
Update: the books at Grandma Nitti's are gone. Gone gone gone. Now, go to Bongo's.
Update: Bongo's
This place is in Gongguan, and I am still awful at telling people how to get there. If you go to Wenzhou St. Lane 86 and then keep walking north on Wenzhou, turning left in the next lane, at the end of that lane where it meets the next street, you'll find Bongo's (you can also get there by walking one block over, away from Roosevelt Rd., from Sai Baba).
Bongo's has a larger selection now that it's taken over for Grandma Nitti's and is worth a visit just for the books (the food isn't really all that fantastic, but pretty good as backpackery Western fare goes).