Glad you found us, Mountainman!
I think it is a great idea to check out the desk in person. If you get any sort of negative feel to it, you can always walk away. If you don't, it sounds like a great peice! If the desk was built by this woman's great-grandfather, the spirit may be attached to her rather than the desk. When and if you get it home, you may find that there is no knocking at all.
I don't think having the item advertised as haunted scared people away on the ebay auction. In the past, that has been a big selling point. What may have happened, is so many people getting burned, thinking they were buying a "haunted" doll/bedroom set/shoes (yes, I saw a haunted pair of shoes advertised), and finding out that it was just a scam to get people to buy something. I've heard of several unethical sellers using that ploy.
We've got several threads on the shows you've mentioned, be sure to check them out. I get a kick out of watching them. There is also one on TLC called Dead Tennants which is probably my favorite right now.