well that says a lot about the person/store you have talked too!!!
Anyway it could be Garden Eel because it does borrow into the substrate. I would not reccomend that you purchase one of these unless you would have a special tank dedicated for s group of them(which could be very cool)
The care needed would simple be a
deep sand bed 8-12inch and thats all they require. You could have a group of them but they can not be in the same tank with other fish cause they might hide the entire day and/or get picked on by other fish. Might not even get enough to eat. The Garden Eel is in the family Heterocongridae so you might want to look them up before you would want to purchase one
The other possibilites could be a chain link(like already mentioned)
jewel moray goldentail moray, there are many to name, it sounds to me that the store clerk was right on thinking it was a Garden eel, since it is burrowing in the burrowing into the gravel.
