From what our accountant told me is, any donation received that is merchandise they donor must provide you with an estimated value. As long as you have that on a signed form, you should be OK. I used the one I found in the tools section of this website, modified it for our group and use that.
Isn't that just like any item donation you would solicit, say, for an auction? I'm not a CPA, but that seems no different. The trick is making sure you don't try to suggest how much the deduction is worth. Thankfully, that's the donor's responsibility. You can always call the IRS Tax Exempt folks at 1-877-829-5500 for clarification.
If I talk to people who are selling merchandise on EBay and, after it doesn't sell, can they donate it to our organization and receive a tax deductible contribution letter since we're a 501c3 organization?