I think it also takes a bit of time to really gauge how the COMC market works. As advertised on these boards, some sellers specifically price and then accept all offers (50% off). Some are willing to accept certain discounts and others very little or none at all.
But I fully agree... if I'm buying and offer a deep discount and get countered with a price that's very close to the asking price, I get the picture and will either accept, counter at a more reasonable discount or simply reject.
As a seller, I gauge each deal, one at a time, based on my own criteria. If someone lowballs me, that's fine -- I'll counter with a higher price. If they counter with the same price or they add only a few cents, I'll just reject outright.
Actually, if a buyer gives an offer that is 50% offer and is countered at a small discount, there's nothing inherently wrong with the buyer offering 50% again -- just to signal that 50% offer is all they're willing to pay. The seller may actually think about it and accept the second time around. Remember, the seller is trying to negotiate the best price for him or herself and may be eventually happy to take 50% offer, but is trying to extract as much value.
In any case, if someone persists more than 2 rounds, the best thing to do is let the offer stay open and let it expire. Since COMC holds the offer amount off your balance, it rightfully penalizes the buyer if they insist on having so many open bids that won't be accepted. So, the best way to get back at someone like this is to have them simply wait and lock up their funds.