I guess this is a good place for this reply, although it will not help you get yours back. Hopefully it can help catch a thief down the road.
There are some great deals out there on used detectors if you look hard enough. I will tell you first hand, because I have 3 top of the line detectors that I got for under $1000... TOTAL!. I am always a bit skeptical when the deal seems too good to be true, so I make sure I cover my bases.
1) Ask a lot of questions about the detector to see if the person knows anything about it. Questions about the operation, features etc. that only a user would know. If the only thing they know is if it turns on or off, be cautious. Sometimes you may hear that it belonged to a friend or a relative...again be a bit cautious.
2) Ask for a serial number. (If it is a recent model White's you can call the Sweet Home OR office and they will verify the owner info. In my case, the seller told me the name of the dealer he purchased it at, and all the info checked out.) If the seller is hesitant to give out this info you should be on alert.
3) If there are missing accessories be cautious as well. When I bought my Excalibur, the seller did not have the charger, stating he could not find it. I was cautious but was also was able to use this as a negotiating point. I told him I was buying a unit that he claimed was a working unit, but I could not power it on. I paid much less than half of that he was asking. As I said in my earlier reply, he knew enough about the detector that I could tell he was experienced with it.
4) Save the seller information: Although I am a believer in "buyer beware" and had no intention of reaching out to the seller, I made sure I saved the info just in case I saw a post like this one in the forums and on CL, and wanted to have the info in case I needed it for the police.
Maybe I should post this in the general forums. We are a small community, but I think we all have a high level of integrity, and by looking out for each other we can keep it that way.