Posted By:
Jason Lgood points...the more communication the better, and if the seller states groundrules like this etiquette stuff up front, it will minimize problems, I'm sure...
I suppose the time limit is a good way to go about it, because it helps us imitate the etiquette of in-person behavior...for instance, if at a card show, someone stops at a dealer table, and expresses interest in a card that's in a case, asking to see it and inspect the back, etc...then another person comes up to the table and immediately says, "I'll take that!" as the dealer is getting it out from under the glass...it would not be acceptable etiquette for the dealer to then immediately agree to sell it to this new guy while the first one is standing there (probably getting ready to punch the dealer in the face)
What I have had happen to me as a second arrival, of course, is that I would ask to see it next if the first customer declines purchase
now wait, do you get one piece from EACH person's collection?! I'm going into dispute resolution! 
edited to add: Joann, I see your points regarding the black/white issue on the presence of conditions, and would agree, but if not for the issue of how to handle the etiquette of timing...If your posts are clear upfront, as you say, then no worries...guess it all comes down to communication...because the internet dealings and email lend themselves all to well to behaving a little more Machiavelian than a person may ordinarily (I have experienced this temptation myself!)