I'll chime in on this one...
If you find someone that is selling stuff on the cheap on a construction site, it's likely stolen. How do I know? Cause I've arrested a few here and there. Take what I say with a grain of salt, though as I'm very cynical.
After speaking with contractors and crooks for 14+ years, I've learned a few things. Like the guy that shows up on the jobsite looking for work and doesn't bring his own tools. He asks if you have any work and you tell him where he can start and he walks off, not wanting to work. He was checking out the jobsite for tools to rip off.
My neighbor across the street does construction. When the season was jolly, he needed to feed his family and buy gifts. Unfortuately for him, he ripped off a church site and got 9 months.
In the city that I work at, if you leave your tools on site - even locked down, they'll get taken. Lots of burgs to remodel sites and jobsite containers. We had one a couple of months ago and the three guys were fleeing the scene in a stolen car when the units started arriving. The K-9 got rammed and she had 11 ribs broken and her pelvis shattered. Still out and loosing retirement credits (like I will be for my surgery next week).
We get lots of downriggers stolen. Hard to track. One guy had sold over 30 of them before we found him on Craigslist and ebay. When we back tracked his sales history, we matched the items to various reported thefts and burgs.
We have some people around here that hang out around grocery stores. When the contractor stops out for Starbucks, the compressor goes missing from the bed of his truck. On guy stashes the stuff within a block, as he's on bike and comes back under the cover of darkness to get the stashed stuff.
We have a couple other groups that I suspect are working Lowes and HD. They carry the Motorola walkie talkies. The contractor shows up to buy materials at 8 am and they are in the parking lot. One guy follows the contractor in and the other guy his the truck for all the tools that are there. They use the walkie talkies to let each other know where the contractor is...
Same scam at Christmas at the mall. People take their presents out to their cars and put them in the trunk so that they don't have to carry as much, then go back into the mall. People sit out in the parking lot and go car hopping to get the new presents that are stashed in the trunk.
If it's a unbelievable deal, ask some questions. If they don't make sense (like a TS55 for a guy that 'works' in construction and he wants $50?!?!).
A few tips for you guys. Take digital photographs of all of your tools and serial numbers. Keep a running list of costs, dates of purchase and so on. Keep the CD or DVD somewhere safe, so that you can show your insurance carrier. That goes for everything in you home or job site. I burn extra family photos too, in case the house burns down. If the time comes to report a loss, it will be much more likely that you'll get your tool back and catch the bad guy if you have the serial numbers (they go in a database).
Cheers,
Rod