The photo in the auction clearly depicted a
Palaemon and reverse image search revealed that the pic is in fact a resized version of
this image of
P. elegans, a European coastal species common in tide pools. I've tried to contact the seller about the origins of these shrimp (wild caught in FL, international trade) and received no reply. I'll give him the benefit of the doubt and assume that this was just a best approximation of what his adult animals actually look like.
I don't think these are actual
P. elegans -- this has to be some other kind of palaemonid shrimp. If they're local, native
Palaemon, they'll quite likely survive in freshwater but I highly doubt they'll successfully recruit there. (That, however, is the norm for
P. paucidens from East Asia -- which more or less fits the description and would be exciting to have in the North American hobby, since they're
apparently quite easy to rear in captivity).
Two other alternatives: they're some kind of
Palaemonetes (conceivably also locally-caught) that's more colorful than your run-of-the-mill
P. paludosus ghost shrimp, or some other kind of freshwater palaemonid with abbreviated larval development (would almost certainly have to be imported in that case, unless we're dealing with an undocumented landlocked population -- all U.S.
Macrobrachium, so far as we know, are amphidromous). Most of his auctions, for what it's worth, are of native animals.
I'm not going to impugn the honesty of the seller, since I haven't seen the animals (or seen his responses to my questions), but there's something a little strange about all this. Though these animals are stated to produce lots of fry regardless of salinity (presumably even in fw), nothing's said about larval
survival rates in fw ... hopefully I'm reading too much into this and they're something new and interesting that function like
P. paucidens.