OBO does not mean "feel free to low ball me for half my asking price".
If at any point, you feel like offering
half what someone is asking is cool, you suck.
It's part of negotiating. And as for offering half the asking price, that's all relative. 10 bucks is a decent offer for a 20 dollar cable or whatnot. True, $75 is very low considering the $150, but the seller was free to make a counter offer/