Maybe some snails have no issues with fluctuations or changes in water/environment however not all are that hardy. Turbo snails are not good with quick change like that. Im sure some are better than others. I guess it depends on the chance you want to take. I QT inverts because while they can't get ich they can be carriers of parasites. Do some research on it. Following are two links you can read so you can make your own decision.
Your salinity level, Ph levels, etc need to be similar if you want to ensure that they live. You may get a good deal online but it does no good if you lose most of them when putting in your tank.
It's only my opinion. I tend to be cautious simply because dealing with flukes or ich is a lot of work and can be an expensive loss as I found out long ago when I first started keeping saltwater fish.