You're the one who put items up without a price, deal with the consequences. Bottom line, for most of the stuff you're selling you could just set a price and say that you're accepting offers through some pre-set date and get the same result. I think it always helps thing the heard of asshats that you have to deal with if you at least set a minimum. Otherwise you have no one to blame but yourself for saying "make an offer" or whatever.
And in the end for stuff like the 501s is it really worth it to you to have to haggle for days on end to get another $15 to $20?
Here's what I'd do:
K501 mint $150
K240 sextett in good condition, w/box $150
HD580 with HD600 grilles, HD650 cables 2 spare head cushions, 1 set of foam disks and a set of left/right fork thingies $150
grado hf-1 mint w/ box and papers, 2 sets of HD414 pads, flat pads and headphile c-pads $450
stax sr-001mk2 mint w/box and papers $200