I bought my Windcheetah second-hand and untestridden, although the owner had done two SRs on it so I knew it was capable, and I'd already read everything going about the trikes (I'd also test ridden a SMGT and decided I didn't like a two wheeled recumbent). A test ride prior to purchase would have revealed that the next size up would've suited me better, though.
I bought my Speedmachine second-hand a couple of years later, having again read everything there was to read, and I had test ridden at SpM at Kinetics before, so it wasn't an unknown. I did quite a lot of miles on it, but had endless knee problems because the massive frame forced me to ride slightly bowlegged. I was nonetheless very sad to sell it.
I rode my Windcheetah and Speedmachine, and just about every kind of bike that came through Laid Back Bikes, for four years before I felt I understood enough of what I wanted my bike to do and to be like. In 2007 I had decided a Lightning P-38 was 'my' bike, but I couldn't test ride any because I simply didn't know anyone in the UK who had one. And while Edinburgh/Glasgow is the epicentre of recumbentry in Scotland it's not London/Ely.
My alternatives were the Burrows Ratcatcher 9 (tailbox! monoblade!) and the Challenge Seiran SL (featherweight! fast!), but neither could carry a camping load or go easily on a train. I also didn't want a Streetmachine because I already had my SpM which despite being fast and very, very comfortable, was pretty heavy.
So I imagined really hard what a P-38 would be like to ride, based on its ergonomics and geometry, talked with Tim Brummer a bunch of times, and finally plunked down $2000 to buy a frameset and have it FedExed over. Yes, it was a risk, but I built it up as my ultimate bike, and I have a ton of miles on it now.
I bought my RANS V2 second-hand, but equally ridden only virtually, went to NY to collect it, rode it to Toronto and then came home with it and did another load of miles on it. I didn't lose much money when I sold it a couple of years later.