That sucks but at least you have a leed. And keep in mind the person selling it doesn't know that you or it does not have a title. I would go and feel it out, verify it is yours, and go from there. If you do think they might have been the one's that stole it (not very likely) then I would just strait take it back! If you cant prove it was yours then they cant prove it was theirs and at least you have pictures from a few years ago. If you don't think they are the one's that stole it, which is probably the case, then you have three options.
One, again just tell them it's yours and your taking it.
Two, if they seem on the up and up you can explain to them that it was stolen, show them the pictures of it, and see if you can convince them to let you take it. You can tell them that if they don't you are going to call the police and regardless if they stole it or not it's still illegal to be in possession of stolen property and it is 100% yours no doubt about it and you'll do what ever it takes to get it back! And let them know you're serious! If they start whining about their money that they paid for it tell them it's not your problem. Ask if they got a bill of sale (which could help you in tracking down who stole it but I seriously doubt the person that stole it gave a bill of sale when they sold it. who knows how many hands it has passed through by now...) and if they did then take it up with whoever they bought it from. If they didn't then guess they're SOL. And if you can't convince them then see option #1.
Three, you could always buy it back. I don't recommend this option but if you're not the kind to stir stuff up or if they don't have CCW in your state or whatever the case may be then this is an option so I threw it out here.
I hope you get it back. I've have many things stolen from me over the years and it's very frustrating having your personal property, specially something you really like, taken from you! Good luck and let us know what happens.