YOU DON'T HAVE TO ACCEPT THESE LIMITS !!! THIS IS A CLASSICAL NEGOTIATION TRAP NEWBIES FALL INTO !!! THEY WILL TRY TO MAKE YOU BELIEVE THAT SURRENDERING TO ANY OF YOUR COUNTER REQUESTS IS A VALUABLE CONCESSION. IT IS NOT. THEY HAVE SET THE OFFER FAR TOO LOW FOR THAT SPECIFIC REASON !!! DON'T FUCKING BUY IT !!!
Guys, you have to learn to play bitches: i) be very patient (time is on your side, you are profitable), ii) talk to other interested parties (i.e. confront the offers to raise the price, and yes I mean multiple rounds; one shark smell blood? get more sharks in the pond! now is the right time!!), iii) don't show you cards, reveal your intentions or commit now, because you are in the process of finding what your intentions are and it depends of theirs too, iv) set YOUR own rules ON THEM whenever necessary (have no hard feelings about it, because they don't have any for you by making a low offer; from your own description, they are not being nice to you), v) never ever buy into whining and strong or manipulative emotions from other parties, they know how to play bitches like you don't (and sometimes, bitches fake friendship, caring and honesty very well; they send the nicest guy/lady, the one that has the highest chance to win your sympathy; it is nothing more than fucking smoke to get a better deal than they actually deserve; YOU did the hard work).
If you don't feel comfortable enough, find an independent professional negotiator with strong AND proven experience who can do that job for you. Discuss the limits within which you are ready to sell and list what you consider acceptable and unacceptable terms. Be honest and candid about it, no early concessions. Then, (re)open discussions starting with the double (at least).
Reward the negotiator with a percentage of the deal. Pay him well, because he/she will obtain a value that you will never ever be able to obtain yourself considering your experience in negotiations.
The hardest thing to understand (or to remember) is that 'no deal' is better than a 'bad deal' in your case. You must be ready to consider that option as the best option, especially if there is no zone of possible agreement between you and the other parties. Negotiating IS PRECISELY about finding out if there is such a zone.
And don't listen to those saying you should turn the offer down because 20% 'is an insult'. They are ignorant morons of no value (really). Now is SURELY NOT the time to quit discussions.
You are in the power position, but you don't act like you know it, or that you know how to play it. Take your time. Buy no drama. No rush. Let your brains run this show, not your hormones.