Apparently so Dan. Since skimmers don't begin to skim for some time after they are set up, they can't just be popped onto a tank when a customer walks in for a demo. The demonstrators also become "used". So I would have to say, based on years of experience, that stores that keep a selection of "demo" skimmers operating must be very rare indeed.
I'm sure that stores that use skimmers would show the ones that they use on their own systems. But even then, how likely is it that the model used in a large multi-tank system will be the model the average hobbyist is looking to buy? If you've got an LFS that does that then count your lucky stars.
There's also the problem that many things affect the performace of a skimmer so it may not perform the same on a store's water column versus yours at home.
I can see how a good store might agree to stick a skimmer (if they stocked it) onto a tank and run it for a week, after which the customer could come in and check it out. That would be great. Bit of a risk for the store. But are you telling me that happens in the vast majority of stores? If it did, and the prices were reasonable, I would be doing a lot more business with an LFS.
I buy salt, frozen food, and some other things from an LFS. But I even have to mail order my dry sand because I can't find decent selection at my local stores. And I live in a + 1 million person market. If you live in L.A., parts of Florida, Indianapolis, or certain other locales then that's a different story. Still, I bet there are very few stores anywhere that offer a large selection of skimmers, within 25% of the cost of mail order, that they are willing and able to demo. That is just my opinion based on my personal experience having lived in six different areas of the Country.