A/C is fairly evident when A. walking through a home during the 1st or 2nd visit, and B. would be certaintly caught during the inspection (assuming one was done). I am frankly a little surprised that you got through the negotiation and came to terms on price etc. before the the buyer realized there was no A/C unit, but I don't know the full details to the situation.
That said, if there is an inspection contingency and you are within the inspection period a buyer can theroretically cancel for virtually anything. Also, as stated below, typically the MLS contains a disclaimer that typically states something to the effect of "The information is not 100% reliable" - basically meaning that it's up to the buyer and their agent to verify everything and not trust the MLS information "blindly".
Best of luck,
If you actually want to sell the house why not work it out so that you attract the correct buyer and move on. Ask the Broker for a break on some fees or something to offset what you feel is an error and get the house sold.
Often times attorneys are the only winners in a lawsuit.
Depending on the cost of the A/C as well, you might consider purchasing it yourself and ask the agent to contribute to some of the cost.
It really depends on the price of your home and how much you would net after buying the system, and if it would fit into you financial situation.
The presence of information doesn't insure your agent is the responsible party.
Did your buyer sign a "seller's property disclosure" on which you stated, "no AC?" If so, I would base my position on this information......
All the best,