I can't give you specifics on the bridge height but I can give you some explanation of why this might be an issue. A 12-string is under enormous tension, especially if the previous owner has been careless and either used medium strings or tuned it above pitch.
The constant tension doesn't usually bow the neck (thanks to truss rods) but alters the geometry of the sound box, including the neck block. This changes the orientation of the neck to the surface of the guitar. Think of taking a deck of cards and sticking a thin plastic ruler into it, so that it sticks out of the deck kind of like the neck of a guitar. If you bend the deck itself, pushing down with your thumbs in the center and pulling up slightly with your fingertips at the top end (where the ruler sticks out) and bottom end of the deck, the ruler's location will change relative to the top surface of the deck. If you had a length of elastic chord (to eliminate slack) running from the far end of the ruler to the distant edge of the deck of cards, the space from the top of the deck to that chord would increase as you bent the deck. This is similar to what happens to a guitar's body (in a less extreme form) after many years of being under string tension that is trying to fold the whole thing in half. As a result of these geometry shifts, the action on the guitar gets uncomfortably high.
To get the action back to a comfortable playing position, the saddle can be sanded so that it sits lower in the bridge. Up to this point, that'll work and things haven't gotten to be too much of a problem. It's pretty common for saddles to be shaved to set action lower. But if the action needs multiple adjustments over time, the saddle can get progressively lower as its height is adjusted again and again.
At some point, the angle at which the strings cross the saddle from the bridge pins (referred to as the "break angle") is too shallow. At first, the height of the saddle just lowers the break angle to an extent that makes the guitar a little less loud and responsive. At some point, the break angle can get so low that the strings actually rest on the wood behind the saddle as the saddle recedes into the bridge from successive lowerings. Early on, the break angle can be improved by slotting and ramping the pin holes. This involves sawing little sloping grooves in the bridge and bridge plate (the chunk of wood under the top that supports the bridge) that run from the pin holes toward the saddle. These let the ball ends of the strings sit a bit closer to the saddle itself (essentially, under the bridge plate rather than merely against it) and increase the break angle. But if the saddle is nearly flush with the wood, this ramping approach no longer works. So, the saddle may be thinned by sanding down its top surface, thus exposing more of the saddle.
The problem with this sequence of events is that it's not the best fix for the original problem. As the geometry of the guitar changes under stress (and Guild guitars resist those stresses better than most), the best remedy is a neck reset. This involves separating the dovetail joint that joins the neck to the body, freeing the fingerboard extension from the top, and pulling the whole neck off of the guitar. The joint is then trimmed and/or shimmed so that it will situate the neck at the proper angle, tilted farther back, when the whole thing is put back together.
This is a repair that we all like to avoid because it's expensive and invasive. I just had this work done on an old Martin tenor guitar and it cost several hundred dollars (although the work is immaculate and you can't tell anything has been done). If this work is done early on, it's a big hit financially for the owner but everything can be restored to the way it should be. On the other hand, if there has been a lot of tinkering with the bridge, the saddle will be way too low for the new, correct neck angle. Furthermore, if the bridge has been thinned, you can't just drop in a taller saddle because there will be too little bridge base left to support it properly. Even if the saddle stays in place, the leverage of the taller saddle in the shallower slot may well split the bridge. So, suddenly you need a new bridge (even if it hasn't actually split but is just too thin to work) in addition to a neck reset.
I'm not suggesting that this is the case with the 12-string you saw on eBay or any other specific instrument. This is just how the scenario can unfold. I worry very little about a saddle that's been lowered a small amount, although I recognize that it may be a sign that a neck reset is inevitable at some point in the near future. Sometimes, lowering the saddle makes the action good and things will stay that way for a very long time. But once the bridge has been thinned, I take that as a sign that things may have progressed further down this worrisome road. I'd want to be sure the neck angle wasn't too bad and would factor into the actual cost of the instrument the price of the work needed to make it right. Again, it might be that the bridge was shaved just the tiniest bit and that things aren't dire, but I'd certainly be vigilant.
The neck angle should be such that if you take a 2-foot metal ruler and lay its edge on top of the frets, the lower edge of the ruler should align with the top of the unthinned bridge. If it falls substantially below the bridge's original top edge, the neck's position has moved upward and needs to be reset. You can also eyeball this by looking down the fingerboard from the nut end, lining up the edge of the fingerboard at the near and far ends kind of like rifle sights. If that alignment has you line of sight below the top edge of the unaltered bridge (or where that edge would be if the bridge hadn't been altered) a neck reset may be needed. If the ruler or your gaze fall 1/8 of an inch low, that's not a biggie. But something like 1/4 of an inch low would suggest that the neck needed a reset.
I hope that I haven't either bored you to tears with information you already knew or raised concerns that don't apply in this particular case but I figured better too much info than too little.
