As others have mentioned, it's important that there is a fair assessment of the market value of the items being donated. Joel's point about the government not looking kindly upon overvalued donations also applies in Canada: the CRA doesn't look kindly upon donation schemes such as "buy-low, donate-high arrangements."
Since nobody has offered up authoritative information for Canada yet, here's something to look at:
3) Gifts in kind of a taxpayer
include capital property, depreciable
property, personal-use property ...
6) The fair market value of a gift in
kind as of the date of the donation
(the date on which beneficial
ownership is transferred from the
donor to the donee) must be determined
before an amount can be recorded on a
receipt for tax purposes. [...] The
person who determines the fair market
value of the property must be
competent and qualified to evaluate
the particular property being
transferred by way of a gift. Property
of little or only nominal value to the
donor will not qualify as a gift in
kind. Used clothing of little value
would be an example of a
You will need to find a charity that would both value the books you would be donating and be willing to issue you a receipt for your charitable donation. Whatever receipt they issue should be in line with fair market value of the goods donated. Assume your donation receipt will be challenged, and keep both:
- An inventory of what you donated (pictures might help), and
- Supporting documentation used in helping assess the fair market value of the goods; the CRA might not accept a verbal "I checked eBay".
Finally, reasonable comparables might be prices for similar used goods, not a percentage of new. Though, if you can't find a price for a particular title in the used market, an estimate consistent with other valuations in the lot would be better than nothing, perhaps.