Lots of great ideas floating through this thread, but your first step is to recontact the police department who took the case. See what they are classifying the crime as (if it's a crime), state laws are different per state, that's why I ask. But if it enough property to push the incident to a Felony status than my guess is someone other than the lower level patrolman is working the case. If an Investigator/Detective has the case, he'll be more than willing to put it to bed, regardless of the level you believe it is at. Meaning if he has a Grand Larceny case sitting on his desk, he'll have no problem sending out one of patrolmen to do a buy.
I know this sounds over the top but they do happen, specially when you've done all of the leg work and all they need to do is be present.
Did you give any sort of deposition? That is key to tell you how much the department you're working with cares, in that deposition there should be enough detail of your property to not only identify it but hopefully make an arrest on it, pending it shows up.
Start with those things.
Get the department involved, figure out what level the incident is classified at, crime and level of offense, make sure you are deposed, tell them you believe you have a solid lead on the person and property and lastly don't do anything yourself.
Even if they do catch the person selling it and can't connect the dots for the larceny, they can at least get your property back and get an arrest for possession.