I'm developing a charitable giving site, and I'd like to use Amazon's
wishlist technology and enable only legit, authorized charitable
organizations to publish their lists on the site.    My primary
question pertains to the donors.  If someone purchases an item from
the list, can they use it as a tax write-off and if so, does Amazon
provide anything to help?   I've been unable to find any documentation
on this anywhere.

The allowed tax deduction on any item is the cost of the item minus
the "fair market value" of the item.  If the purchasers are paying the
same amount through the wishlist as they would through Amazon's normal
website, they would not be allowed to take any tax deduction.  For
example, on one PBS station's website:

"WGTE Public Broadcasting is a 501 (c) (3) nonprofit organization.
Your contribution to the stations is fully tax-deductible minus the
fair-market value of any gifts you receive."

(http://www.wgte.org/members/memberfaqs.asp)

Thanks for you comment.  

I think this is a different situation than the one with PBS.  Visitors
to our site would be purchasing gifts for the organization, from their
wishlist.  In other words, the people buying the items wouldn't be
receiving any type of gift.

A bigger part of my question is whether or not Amazon provides the
ability for the buyer to obtain documentation, to include the
organization's tax information, the amount of the purchase, and
whatever else necessary for the donor to have on file for their taxes.