I am going to put a motherboard on eBay with the Windows XP OEM copy and product key that I originally installed on it. This copy of windows has only ever been installed on the motherboard I am selling it with, so the buyer will have no problems installing it assuming they use the motherboard/OS combo that I sell them, right?

Also, I still have that old installation of windows sitting on a backup drive that is currently hooked up to my new computer that is running windows 7. It doesn't do anything and I don't boot off of it, I just haven't deleted it yet. Probably a stupid question, but THAT won't cause issues either, right?

Not positive on this one, see if this:

http://technet.microsoft.com/en-us/library/bb457054.aspx

has any answers for you.

Here is what I suspect/experienced. Usually you get about 3 installs on 'different' hardware before online activation will no longer accept a product key, forcing you to call and deal with the hassle of telephone activation.

I believe the motherboard s/n is only 1 part of the activation hash key thats sent to the server, if every other component is different then it's likely that attempting to activate XP online will result in the server processing it has a new install on different hardware. So it sorta matters how many previous times you installed XP with this product key.

Hmm. Well, I probably installed WinXP on the same hardware about 3 or 4 times, including the first install back in 05 and 2 or 3 reformats/reinstalls. It was always on the same mobo/CPU/RAM and this exact CPU/RAM/Mobo/product key is all being sold together.

Not sure who would buy that, but yeah it should be no trouble.