Let's see if I understand this right: you want to return a mobo, cpu, and ram because you can't get the mobo to work with the rest of your system.
Well, if they gave you a written warrantee that it would work with your specific hardware setup, then you might have a leg to stand on. Otherwise, you're SOL. Personally, I think it's mighty nice of them to offer to buy it back at all. You have absolutely no right to expect it, unless it's a published store policy. After all, there is nothing faulty about the hardware EXCEPT that it won't work with the other stuff you own. Know what ASUS will say? "Why didn't you research the compatibility question before you purchased?". And they're right- they DON'T warrantee that their mobo will work with every hardware combination out there. They don't warrantee that it will work with ANY hardware out there.
Read the fine print next time.