I've got to admit, the internet is not where I go to buy books. Used bookstores, preferably disorganized and cluttered, are far more interesting. That said, there are some folx out there selling hard-to-find and used reading materials on the web, including some stuff that is really worth taking a look at.
The following links are to sites I have found offering unique selections of books. Check them out, and if you see anything out there that you think I would be interested in, let me know!

Stoop Sale Books : Hundreds of books; while some are clearly garage sale material, the vast majority are nicely placed on the far-left side of the political spectrum.

Everglades Books Company : This site belongs to a collector of queer fiction, especially pre-Stonewall stuff.

  Conchar Books is a radical used bookstore in Wales.

If you must patronize Amazon.com, I strongly suggest that you go through Michael Stec's radical bookstore in conjuction with Amazon. That way at least somebody with good politics will make a little money from your purchases.