Resale value in saddles is almost totally subjective because fit for both rider and horse is so variable. On the whole, the established higher end leather saddles tend to do better on resale when they are well maintained and there is a “market” for the particular seat size, tree and style you have. Well-made leather saddles can last for a very long time. I have an 18 y.o. Albion Original Comfort special order with 19" seat, wide tree, short flaps and extra deep gussets. It’s worth a fortune to me, and probably to someone built like me, if I ever decide to give it up. It would come down to how long I’m willing to wait for the right buyer to come along.
It’s almost impossible to say what has the most appeal. Custom is one of many brands that do well, but there are so many others. If you search this site you are likely to find someone who isn’t happy about a particular brand even if the rest of us love it. I have a young friend with two older saddles, a County and a Stubben, both in terrific condition. She got great deals on both of them because the owners really wanted to sell. Willing seller, willing buyer.
Looking at the websites of some of the major consignment saddlers would give you a reasonable idea of what is out there and what the price range looks like. I’ve done business with Pelham Saddlery in New Hampshire for a long time, and others will give you the names of similar shops. I do agree that second-hand is a good option. But I doubt you’d be able to recover most of your purchase price on any saddle - new or used - if you decide to turn it around quickly because you’ve decided to go back to trail riding.