The auction lists them as Ilyanassa obsoleta. These are temperate snails, they don't usually do too well at warmer temperatures. Typically I'd expect this kind of snail to spend quite a bit of time buried in the sand (could be wrong though, I've never kept them) -- if they're not, and just lying on the substrate, they're probably on the way out. Hermits are opportunistic, they may just be preying on weak or dying snails.