If you don't think $2160 is fair, then say "no". Unless you really want to sell, there's no reason to counter offer. If it isn't worth more than $2160 to the potential buyer then it's worth more to you than them.

If you want to sell, then decide what you think is a fair price and negotiate your way to it.

Next time, consider having a buy-sell agreement, vesting, options rather than shares, etc. irrespective of which side of the transaction you are on.

Good luck.