Funcitypapa: It is a good question and one I will obviously ask more often now.
OTOH, the mere existence of a prior cert is hardly proof that a stamp remains sound to this day. Accidents do happen ... It might be wise to get a re-cert anyway, so in that case, my initial thought was: why worry about whether you get one with the stamp in the first place?
My second theory was only to buy pretty much "faultless" examples, from respectable sellers with good feedback.
I guess I have learned the hard way that even when an apparently respectable, established
ebay seller, specializing in rare/high-value material, says unequivocally that a stamp is "perfect", "faultless", "a gem", etc., that may still not be true. Frustrating, but I am not sure that it was entirely foolish of me to assume that such a seller would not risk their reputation by selling such faultly items.
In any case, I'm still cutting myself a little slack -- I have only been focusing on US for under a year, and my vast GB collection has been built up over 45 years without these kinds of investments.
BW was kind of enough to recommend a couple of sources for the future, and for now, as I said, I am focusing elsewhere in my collection (precancels, here I come).
Btw, you reminded me that I wanted to share some other info to the group in general: not all of the stamps were in fact previously uncertified -- one of them had a small pencil mark on the back that turned out to be marking a rather large thin from when BW certified it before, and that cert was neither mentioned nor provided to me.