I wouldn't be in a hurry to sell cheap.
Fan engagement isn't a hot and developed phrase yet, nor a highly-searched term; yet it's just being born as a social marketing catchphrase.
Marketing catchphrases can die a quick death... or they can explode. Since this term refers to music media, sports media, movies, celebrities, etc and by definition involves info/techniques about how to 'mine' more involvement (read: money, interest, signups, buying things, buzz) from fans, I think it's a strong contender for quickly becoming one of those more optimistic marketing catchphrases.
I can't see it ever being worth like $100K even if it does take off. But if I had this name I wouldn't let it go for peanuts like a few xxx.
Right now, its value IS peanuts, because it's one of those too-soon-to-tell names that could skyrocket or could smash and trash. But my feeling is that it is going somewhere, and it's too soon to tell just how big.
My guess is your offer is from someone who is forward-thinking and is lowballing you, not according to your domain's present worth but according to the possible writing they see on the wall.
As a reseller I'd pay you $200 for this name right now. If your offer isn't considerably more than that, I'd decline the offer.
Lemme see: if I owned this name, I might let it go for $2K. Less than that, and I'd hold on to it and see what happens with the phrase over the next couple years.
That's my appraisal
Oh, how much was the offer, if you don't mind sharing?