All i know is that temperature for these guys can be a problem, need to keep low i believe, they dont like warm water. Im sure someone else will chime in.
Have you done some research??
Try to find some info on another site. They are trying to sell the item to you, so they might be exagerating the temp that they can live in.
I do not think that they will make your tank water stink. If they die, they can, but that goes for every other animal in the tank. If a snail dies, it can make the water stink. If a large fish dies, it can make the water stink.
I've had them in the past, but they don't seem to live too long...buying them from the lfs may be expensive, I've always bought mine from the fish market for cheap.
i know with freshwater clams most people say not to get them because you almost have to dig them up every few days to make sure there there people have had them die in there tanks and cause problems because they rot under rocks/sand idk if its the same for salt