"What can I possibly hope to look for when inspecting the sails in this guy's garage?"
You'd really need to be familiar with sails to make a reasoned evaluation. I'd suggest calling a couple of lofts, or stopping by, and telling them up front you are looking at a used boat and won't be able to buy new sails THIS season, but you need to understand what "good" versus "trash" is, could they show you some sails and educate you a bit? Odds are they will say yes.
With any sail, you can't really see if it is holding shape unless it is hoisted. You can tell if the sail bellies out too far aft. But with any dacron, once the coating has gone and the material starts to get soft enough to sleep on--that's going to be stretched out as well. With stitching (on any material) it is easier to look at the sail hoisted, you'll see daylight coming through where stitching is wearing. Laying down, that's harder to see.
Mold damage, internal in layered sails, is also something to look out for. Delamination will make composite sails look "crinkly" as it is starting, and you'll see the layers peel and separate once that really takes hold. There's no stopping or patching that. With Kevlar and some of the other composites, lofts will also tell you to only ROLL the sail, never fold it, never step on it, never flake it, because that breaks the fiber strands and the sail loses shape quickly after that.
But the best way to get an idea, is to have seen and held sails in your hand, to get a better feel for what "degraded" is. Sometimes there are bargains in used sails, i.e. someone may buy a storm trysail or a particular jib/genoa and simply never use it. More often, used sails are used up, stretched or otherwise just not much better than bedlinens. Yes, you can sail with them, but once you've sailed the same boat with sails that have proper shape--you'll be amazed at the difference it makes.
If you can call a loft and arrange to come by, do so. Simple bribes like coffee and donuts often get people to be very happy to take a break in their day and talk to strangers.[g]