Seen a lot of posts like this come up lately, people getting axed from pugs for no reason (they claim) because the groups are just selling the kill and using them for dps in the mean time.
There is a really easy way to make it an actionable offense, or to sniff out whether the group is a legit run, or they are using you.
Pay them. At the start of the run, tell the leader that you won't run with them unless they allow you to pay for the spot. This means a legitimate trade going up, currency changing hands, and Blizz has something to act on.
Make sure you explain that bit, if you're worried. The groups that are just going to kick you after NB will probably just kick you right then and there, saving you time and effort. If they refuse to allow you to pay, just leave. If they were normal human beings who just wanted to run the dungeon, they would accept your payment and move on. Even if they were really nice people who didn't need your gold, they would understand the implication you are setting forth that you are solidifying your spot in the group, and that there will be an actionable offense on their part if they were to kick you before the run's completion for no reason. If you explain that when you pay them you will have reason for Blizz to take action against them if they unjustly kick you, they'll either kick you because they know they're going to do it after the boss, or they'll accept your payment because they can profit off your paranoia.
Blizz keeps telling you people that there is no "legitimate trade" going on because there is no trade window going up, no currency exchanging hands. So make that happen. It doesn't matter if it's 1g or 1k. Make sure the leader lets you pay them to make the spot in the run legit or you'll just leave and they can find someone else to con.
It's a sad state of affairs when advice like this is handy, that we can not trust other players in the community on a basic level =/
EDIT: Forgetful me, can't cross-realm gold, etc. So this only applies to runs on your own server. Still works for that though.