To me any offer less than 75% of asking price I consider a low ball, tire kicker and don't bother responding. If they're really interested they will send a better offer. If not, no loss to me, they weren't really interested any way.
It's just like someone fishing for the biggest sucker with a price that is twice market value. They aren't any more interested in selling than the lowballer is in buying.
I've never sold a game that the negotiation started at less than 75% of asking. I used to respond to those but don't bother any more.