Do you have any problems with handling/steering now??
If not, don't waste your money, and in fact, if you do have problems sort them out and don't just hide them with a damper. They are only really needed to give temporary help when riding with ridiculous loads an the back etc on bad roads, or for helping with a sidecar attached.
If you're still hanging out to have one, there are from memory about three different types, a hydraulic and two friction types, which is this one? Send a link of the eBay so we can see. Also be wary of SH hydraulic units as they wear and lose ability to damp so are next to useless if worn/leaking.
Bill....................;-)