If they file a complaint they will get their money back because paypal will just refund it, even if you dont want to let them, you will get a negative feedback.
About 6 months ago I sold a stethoscope on there that had the ability to amplify itself with a push of a button. I said in the auction it did not work and I said in my auction no refunds. Guy wins it, gets it, says the amplification doesnt work and wants a refund. I say no because I said in the auction the amplification didnt work and that I said no refunds. So he files a complaint and paypal refunds him his money out of my account and closes the case. He also left me a negative feedback because sellers do not have the ability to leave negatives for buyers. At the time I had over 1300 positive feedbacks, 2 negatives, the guy who bought it had 3 feedback total and they still didnt care I had sold over a hundred things this year alone and all were positives.
I sold a broken laptop on ebay also last month and it said in the title broken in caps. Person wins it, emails me to say they dont want it and they never pay me. So I file a complaint with ebay and 4 days later they refund my like .35 cents for listing it and do nothing to the seller, they are still running around with no negative because I cant give them one for not paying.
Ebay/paypal do not care about sellers because if someone complains they will automatically take the buyers side and punish you regardless. Ebay is ok to buy on but it sucks to sell on because you can get complaints filed against you for anything and have nothing you can do about it except suck it.