Depends on the books and magazines. Some books can be worth considerable sums (if you find the right venue to sell). Most people put up for sale online are pretty run of the mill and common. Those are mostly worth only a couple of bucks, if you can unload them. Titles such as John Naka's Azalea book (and his Bonsai Techniques Vols 1 and 2) can bring some money --more if they're autographed. Bonsai Today magazines are great IF you have the earliest numbers--1-10 are popular and people pay for them. Other western bonsai mags (other than the ABS Journal and some BCI) are rather worthless...Japanese bonsai magazines, however, are always worthwhile because of the photos.
FWIW, I have found a lot of excellent old bonsai books in a great second hand bookstore here in the D.C. area. Apparently, the place has somehow tapped into the local bonsai community and buys up whole estate's worth of some pretty nice bonsai books and Japanese bonsai magazines.