At least as far as Scubapro/Uwatec is concerned, no authorized dealer is permitted to sell over the net. That means that any on-line dealer selling Scubapro/Uwatec is either not an authorized dealer (e.g. LeisurePro) or is about to lose their franchise because they are violating their dealer agreement (as was the case with scubastore.com). Non-authorized shops are dealing in what the industry knows as "gray market" goods. These are items which were purchased by the on-line store from a legitimate dealer at some greatly reduced price, and then are resold by the on-line store. Scubapro spends a very large amount of money each year buying gray market goods off the internet just so they can then track the serial number and then go shut down the dealer who sold them.
Scubapro/Uwatec will not provide warranty service on any items which did not come from an authorized dealer. For something like a BC or a regulator or a computer, this is a very big deal. Warranty returns are not at all uncommon. Quite often, Scubapro returns a brand new item in response to a request for warranty service on a used item.
One other point worth mentioning abuot gray market goods is that quite often they are obsolete versions. I've seen some very strange versions of Scubapro regulators come in for repair, and when I check they invariably turn out to have been on-line purchases. Needless to say, these regulators don't get the free repair parts for life service that items purchased from a legitimate dealer receive.
