'Demo' bindings are the type of bindings used by rental shops. They are adjustable, to fit a wide range of ski boot sizes. The range of adjustment can be 100mm or more. If you're buying your own skis, the bindings will be mounted to fit your boots. They may have a small range of adjustment, but it won't be much; +/-20mm is common.

So if you're buying used skis on ebay, you'll want to know if the bindings will fit your boots. Not necessarily an easy question to answer. Here's why: Boot size is a measurement from inside the boot...inside the hard plastic shell, and inside the liner. It's basically a measure of your foot. Bindings are set according to the boot's sole length, which is measured on the outside of the boot. It is a measurement of the outside of the plastic shell, from toe to heel.

Here's the catch: THERE IS NOT A DIRECT CORRELATION BETWEEN BOOT SIZE AND SOLE LENGTH.

Why? Different manufacturers have different size soles on their boot sizes. For example, my older Nordica boots, size 10 1/2 (Mondo 28.5) had a sole length of 295mm. My new Salomon boots, also size 10 1/2 (Mondo 28.5) have a sole length of 322. Much larger sole, same size inside! 

So if you ask me "Will these bindings fit my size 10 boots?" I won't be able to answer that question. I'll need to know your boot's sole length. You can find your boot's sole length embossed on the side or rear of your boot's sole. It will be expressed in millimeters; example "295mm"