But how do you guys go about buying these revolvers and knowing you're getting a decent deal?
... Triple Lock, 1st, 2nd, 3rd and 4th model 44s, a 1917, a 38/44, etc.
You have good taste, my friend.
For over a decade my entire S&W collection was a 640-1 for concealed carry, a used 29-6 Classic 6.5", and a 6.5" pre-27 shooter from the estate of a well-known dealer. My "collecting" interest started after buying a copy of the Standard Catalog of Smith & Wesson and reading "Sixguns" by Elmer Keith. I studied the pre-27 and realized that I had something special - a set of Coke bottle stocks. From then on, about 2007 or so, I was all about prewar, transition, and pre-model N-frames. But I had the same questions that you do.
I visited gunshops, pawn shops, and gunshows, talked to a few gunsmiths, looked at online auctions and read forum discussions about the models I was interested in, and began participating in this forum. I joined S&WCA and went to the Tucson symposium. I reached out to individuals and dealers and let my interests be known. I asked for help and got it - in abundance.
My collection is shooters mostly, because that's what I can afford. But along the way came a few nice collectibles with all the goodies. Once in a blue moon I'll find a real gem in the weeds of the local online classifieds. Mostly I rely on this forum, a few good dealers, gone-broker and a couple other sites (for price research), and a personal network of like-minded enthusiasts. In my circle I'm the "Smith & Wesson guy". Though I'm no expert and certainly not a gunsmith, I'm pretty good at vetting and research. I just know a lot of people who know these things, I guess.
I look for original and correct first, then mechanical function, then wear. Then I consider the age and relative scarcity. "A decent deal" is a relative term. If it's somewhat rare, like a transition 4" 44HE 3rd Model, or a pre-21, and I need/want it for the collection, then the price is driven by scarcity, condition, originality, and documentation such as a historical letter or provenance. A more common example, such as a prewar HD, 1917, or 44HE 2nd Military can be a "decent deal" if it's original condition with honest wear, and intended/bought as a shooter.
My collection is OD's/HD's and .44HE's for the most part (2nd, 3rd, 4th, and a Model 21). Triplelocks are somewhat new to me. I have two, a .455 Military and a Target. Both are refinished, the .455 is now nickel (and pretty decent), and the Target is a factory refin with a few mechanical mods (early 1950's-ish) for serious target shooting. They aren't original, but both are somewhat rare (I don't see them very often) and both fit the context of my collection of shooters. Both were bought after consulting with dealers who know Triplelocks. They weren't cheap but were certainly a "decent deal".
Two other examples: A .44HE 3rd 5" transition, what finish remains is now plum, lots of external wear and chinks in the cylinder, and a ring in the barrel (not externally visible). BUT with original matching magnas in rough shape. I bought it relatively cheap, had my 'smith go through it, and DWFAN restored the stocks. It's now a favorite shooter and quite a conversation piece. To me it was a "decent deal", to a collector, probably not. The final example is a 1951 pre-23 Outdoorsman, original condition, matching stocks, next to no wear, a time capsule and superb example of the craft of that period. But the barrel is bulged just behind the lazy "&". Zero collector value but still all original in otherwise good cosmetic condition. I bought it cheap, sent it to my guy, and it's as accurate as any OD you'll find anywhere. That was a "decent deal" in my book too.
Good luck, learn all you can, and jump in with both feet!

Some nice old shooters.