depends on what you want to do. VNC works for basic screen/keyboard/mouse viewing and control. There's a basic server built in to the ARD client included in OS X, and the OSXvnc server shareware(?) works fine and has more options. There are a bunch of clients, I prefer Chicken of the VNC. You can tunnel it through SSH for security.
Timbuktu (sp?) provides a commercial product that competes with ARD, but I don't know much about it, haven't used it since the mid-90s.
As far as that ebay posting goes, who knows. There's all sorts of reasons, legit and otherwise, to sell something below retail. ARD is a good, full-featured product, but may be overkill for what you want. My wife uses the old OS 8/9 version (Mac Manager, I think) in her school's lab, and it works fine, other than having issues with being so out-dated.