150 USD for a well condition PRS-500 without a AC adapter is still a very good price. Keeping in mind that the above suggestion is quite valid for this model, since it can not be charge through USB cable if the battery is completely drained.
I have been stalking eBay for the past 2 weeks or so noticing that used PRS-500 seems to be in demand. For example,
this particular auction (which I lost) end up ending at 200 USD and it does not include a AC adapter either.
Or... even
this one, where it was a gamble on wherethe or not the unit just merely needed a AC adapter to charge it. I
now hope whoever outbid me did not end up with a 100 USD paper weight...