First, I don't buy horns to resell per se. I buy a sax, generally a reputable vintage sax, to see if when rebuilt I will like it better than my favorite sax. If it doesn't make the mark, I donate it or resell it. Obviously, by donating a sax I don't make a profit. It turns out that the same is often true with reselling a saxophone. If I bought an old saxophone, cleaned and adjusted it, and resold it, I might make a profit, but that's not my hobby. My hobby is to get it playing as best as possible. That can't be done on old mismatched pads, in my experience.
The often seen statement describing a saxophone that is for sale is "playing well on older pads." Or the seller says "some pads may need replacing in the near future." I guess replacing a few pads to keep the horn playing is fine for a grade school student whose parents don't think that the saxophone will interest the student for long. Or for somebody on a really tight budget (although new pads are cheap if you do you own sax work). But for all other situations, trying to guess which pads should be replaced is simply not worth it. From the used horns that I have seen, it is only when the pad is really, really bad (and really really obvious) that it gets replaced. That is like waiting for the core to show on your tires to decide that they need replacing.