I love revolvers.... The big and small dealers who specialize in hi-cap black auto's black rifles, and tactical double barrel shotguns with optics... Will from time to time take in a revolver in trade. IME they will usually be quick to get rid the offending thing....
Well I am not going to argue with obviously more knowledgeable guys like Chris, but I just did a quick check on GB and I only saw 10 Model 15's for sale. So I don't think they are that common. They are older guns after all
Well I may not be Chris, and therefor I may not be knowledgeable, but there are actually *62* results for a "Smith and Wesson 15" on gunbroker. Some of those may be a "pre 15" but still its a K38 Combat masterpiece. I am not sure why you couldn't find all 62, and only found 10. Perhaps you searched "S&W 15 38 special" which is more specific. We already know any model 15 is 38 special.
As far as "not common because they are older guns afterall" they most likely don't have a high attrition rate, since many PD revolvers were 357 magnum, and many were SS which would be a model 67 in this case. Of course many PDs carried model 15s, but probably less than you think. Many other PDs didn't carry revolvers with adj sights. Many carried Colt, etc. They debuted in the 1950s, around 1954 IIRC so its not that old of a model or design. The gun was made until 1999, and actually made since then in limited quantities and in the classic series. In other words, I would expect the model 15 to have a high survival rate. In addition, its not a magnum caliber which accounts for some losses in handguns.
Model 15's are probably one of the most common S&W's ever produced. They were used by a lot of police departments until the switch to autos. It should be one of the easiest of guns to find a decent deal on. I wouldn't consider $550 a "deal" by any stretch.
I also don't consider gun show prices to be a proper litmus test. I can't remember the last time I bought a gun at a show. Most have been ridiculously overpriced for the last ten years at least. Now that there are fewer and fewer ignorant sellers, most are taking their cue from Gunbroker and asking more than the stuff is worth.