Yeah, I see them all the time at the LFS. I agree, I always think they look like they are dying, even though I know perfectly well what they are.....it still kinda disturbs me.
I have three of these guys, and I love them ! You won't see them too much during the day, but they'll always come out at feeding time. They love Hikari Sinking Wafers ( and probably most other algae wafers too).
Best to get them in groups of three or more, as they're social like corys.
I have seen both synodontis nigriventis and featherfin synos listed as "upsidedown catfish" in my LFS. I have both in my 55 and love them. Mine are totally nocturnal and only come out during the day to grab some food and zoom right back to their hiding spot. Not sure if this is normal, but they school together at night. They're really cool to watch.