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From what I know bobtails don't get too large with a max size ranging from 2-8cm mantle length (correct me if I'm wrong). So it means if the one on sale is rather large, it could be near the end of it's life. They're nocturnal and like to bury in the sand.
I'm sorry I can't really help much but try doing a search on old threads and see what info you can dig up
Most likely it is euprymna scolopies, which is a species from hawaii. They only reach ablout 1.5 inches and are tan but can show some red/brown colouration. They only live about 6 months so if they are already adults it will probably only live a month or two. As far as care they do not need a big tank a 30g would be big enough for one or two. However they need a fine sand bottom about 1inch deep to hide in. They are nocturnal so they usually only come out at night. As far as food we feed ours small live shrimp.