sell off vs sell

sell off is not a noun while sell is a noun.

Word	Noun	Adjective	Verb	Adverb
sell off	No	No	Yes	No
sell	Yes	No	Yes	No

As verbs, sell is a hypernym of sell off; that is, sell is a word with a broader meaning than sell off:

sell off (noun)	sell (noun)
	the activity of persuading someone to buy

sell off (verb)	sell (verb)
get rid of by selling, usually at reduced prices	persuade somebody to accept something
	exchange or deliver for money or its equivalent
	do business; offer for sale as for one's livelihood
	give up for a price or reward
	deliver to an enemy by treachery
	be sold at a certain price or in a certain way
	be responsible for the sale of
	be approved of or gain acceptance