There is a lot of stuff out there, so it sort of depends on the model & manufacturer. Generally, they are identical bindings with an adjustable track underneath or in place of standard mounting brackets. The track might be made by the binding manufacturer, or it might be a third-party add-on.
As far as performance goes, the track usually adds height to the system, so a demo binding skis not unlike a standard binding with a riser plate. That can be a good thing since you'd usually pay extra for a riser plate, or a neutral thing in that a riser is not necessary on a wide-ish ski--but it won't hurt much. Some people might worry about the demo track affecting the stiffness and flexing of the ski.
A more important performance aspect is that demo bindings are often beat to s***. In any case, but especially if you're a non-techie and/or you are buying these from some questionable venue like eBay, plan on spending a few more bucks to have the bindings inspected, set, and release-checked by a shop that is certified by the manufacturer of the bindings you get.
Be prepared for the shop to tell you they won't work on them. They probably won't say that, especially if the bindings are only a few years old, but it might happen. If the bindings are no longer indemnified by the manufacturer, working on them would be a legal/financial risk for the shop that they cannot afford to take; it doesn't mean they just want to sell you new bindings.
You would like both the toe and the heelpiece to be adjustable, especially if you have very small or very large feet. "Demo" bindings usually have this, "rental" bindings often don't (instead they may have a fixed toe, adjustable heel). Few people selling stuff on eBay admit to selling "rental" equipment, but there is lots of it for sale there... Caveat emptor.
Ideally, you would buy demos from a shop you know and trust, who will guarantee the bindings, who can tell you how many days they've been skied, and who can set them and test them for you. Personally, I like demo bindings because it makes it easy to share.