Re: For Sale section etiquette
I don't think that there are any hard and fast rules re: situations like the one you find yourself in. I try to always give folks the maximum benefit of the doubt based on nothing more than a misplaced belief in the goodness of human beings and the fact that in this day/age of instant comms, @#$% happens and sometimes we can't always been ever-present online to respond.
Here's what I'd probably do. First, do your homework on the seller. Is there any sign that the seller has received your PM (should appear toward the bottom of the PM with "Unread by: _____." If it says "None"...the seller got your PM and I'd say was ignoring you or otherwise indicating his refusal to proceed with the sale. If it is still unread...check out the seller in the User List tab to see when he was last online. If he's been online since you sent your PM, then again...I'd say you are getting the run-around or otherwise being ignored. Also see if any other issues with the seller have come up in the past re: slow comms, problems with selling/buying stuff here on the Hide, etc. If so, then again...well...you get the picture.
With all of the above noted, I'd say that a couple of days is probably a reasonable "wait time" for a response. If nothing, then I'd follow-up with the seller by PM (and probably in his for sale post to alert others to comm issues) by saying "Despite my previous PM indicating that I would purchase your ________, and other attempts to reach you since that initial message, you have failed to get in touch with me to acknowledge my PM or to otherwise complete this deal. Therefore, I'm withdrawing my offer to purchase your ________ given your lack of communication and apparent unwillingness to proceed. Best of luck with your sale."
Anyway...it is ultimately your call and you should base it on what you are comfortable with doing. Good luck!