I've sold probably 10-12 used tandems over the past 15 years or so (either upgrading, changing sizes, or simply fixing up and reselling), and similarly have bought quite a few used ones, too. I sold two this year, one locally and one that I shipped. I'd say the current tandem market is fairly priced, perhaps even a bit lower than before, compared with the crazy prices single bikes (used) are bringing due to Covid. Tandems are still specialty items that the average person is not buying. When I sell a used tandem I take a TON of photos so the buyer, either local or remote, knows what he/she is buying. Usually I'll talk to a non-local prospective buyer on the phone to answer any questions, or if I'm buying will always call the seller. You're never going to be totally "safe," but you can usually get a pretty good feel for whether it's legit or not. And use PayPal Goods and Services (even if you have to pay the fee) for the safeguards it offers.
Bicycle Blue Book is a joke IMO and it annoys me when potential buyers e-mail me and berate me for not adhering to the silly, low prices it lists.
Many sellers have no idea how much to list their tandem for, resulting in a wide disparity of prices for basically the same tandem. If you are in an urban area, or reasonably proximate to one, you'll have an easier time selling for more $$ because the shipping thing freaks many people out. Small and medium-sized tandems seem to sell most readily, as they fit the widest range of people. A large or XL tandem will be harder to sell. I sold a very nice XL Santana this spring and it took a while and sold for much less than it was worth (IMO), but mainly because the cohort of people that fit an XL is pretty small, and I wasn't willing to ship it. I also sold a medium S&S coupled Santana, which I was willing to ship (due to it being coupled). It sold in just a few days for more than double the price of the XL tandem, even though it had similar specs (actually, it wasn't as nice) apart from having couplers. Due to back surgery, I recently had to literally "downsize" from my two medium-size tandems to small-size tandems, and I can tell you its VERY hard to find a good small-sized tandem :-(
There are a lot of really nice "old" tandems with 9 speed drivetrains out there that are still perfectly usable and I wouldn't hesitate to buy (actually, I think 9 speed is the sweet spot for tandem drivetrains). I bought a year-2000 26" Santana S&S coupled tandem (size small) last spring; even though it was 21 years old it was in excellent condition and had a 27-speed drivetrain with brifters. Some might hear "21 years old" and roll their eyes, but it's really not that much different from "modern" tandems with 11 or 12 speed drivetrains.
I've bought two coupled tandems off Craigslist from other states, which had to be shipped. In both cases the sellers hadn't considered shipping, but when I asked were willing to drop it off at a local bike shop to get shipped and packed, which I set up and handled. There are ways to ship a non-coupled tandem, but it takes a bit more work on the seller's part and costs a bit more than a coupled tandem. If you want to buy a non-coupled tandem that's $$$, consider flying out and buying it in person and then bringing it back on the plane with you. For a high-ticket tandem tandem of your dreams, plane tickets are a small additional cost for being able to test ride and do the deal in person. Work with a local bike shop to get it packed (put two regular boxes together) and pay the surcharge to take it home with you on the airplane.