- It's none of your business.
- It could be risky, if the guys are mafia, or otherwise shady.
- Perhaps you could help, though.
I advise looking at the law. Here, for instance, if a salesman comes into your home and you sign, you have 14 days to walk out of the contract, because there were too many cases of salesmen putting their foot in the door and strong-arming the elderly into what is, basically a con job.
This is not the case if you sign outside of your home, or in a shop, because in this case it would be assumed that you went there of your own free will with an intention to purchase. Thus there is no grace period.
Thus, the usual tactic is to put the foot in the door, then talk people into having a ride to the shop "Let's go into the showroom! You must see all our nice granite countertops!" (note: quote from actual experience).
They will then bring you there using their own car so you can't get out until you sign for a €30000 kitchen (actual price also from experience, this happened to my parents, who didn't sign obviously).
So, know the law. If you know that the "client" is entitled to a 1-2 weeks grace period where he can walk out of the contract scot-free, then you can simply mention this to him after they leave, no need to interact with the con artists, just catch up with him in the street or in the toilet.
And just in case, if they really feel like con artists, write down some information and perhaps talk to the cops. There are also associations of victims of such shady deals which would be willing to help.
Now, I have applied a more direct approach in the past:
Walk to the table, sit down, and say "Hey, this looks like a good deal, I was looking at something like that, mind if I read the brochure?" Try to sound genuinely interested, be a good sucker, enthusiastic at first, then as you read the fine print, casually criticize it, "Hmm... this clause looks like it could be really expensive. Hmm, wow, on second thought this deal doesn't sound that profitable..." (ask the victim what they think) "Of course I read the fine print, I'm a lawyer."
Yes, I've done this before, they were selling Time-Share appartments. Never buy these, it's a really bad deal. The salesmen recruited us in the street, and we went along since we were curious. Obviously I knew what I was getting into. They brought us to visit the premises, offered drinks and toasts, then the "clients" were corraled into an open-space office like cattle and the professionals were let loose on them.
Game on! I made sure the two couples in the cubicles on the left and right overheard as we offered insight on the fine print. After they (promptly) left, the guy in front of me (who was beginning to sweat) left and his boss sat down instead.
Gotta give him credit: he eyed us for like two seconds, then paid for the taxi back home.
Summary: Two couples saved from a con worth north of 30k, plus all the others whom I made sure to whisper to during the visit, "You know timeshare is a mOney sinkhole, right? We're just here for the drinks."
Do not do this if you cannot mentally disassemble a salesmen' argument in real time. But then again I used to have my office next to the same kind of guys, so practice makes perfect...