I don't think I ever heard of bribery in the congregations I was in. But there are at least a few people that I would not be surprised to hear if they had done something like that. An organization like the JWs is ripe for grifters. I recall hearing a few stories (mostly told as warnings) of people who would go to congregations pretending to be an active JW or an interested person, and would ingratiate themselves and take advantage of people to get money or goods/services out of them. Once people started to notice that these persons were taking from everyone and not helping anyone, they'd disappear and find another congregation to ransack.
I don't know if any of those stories were real. There was probably a bit of truth to many of them, but my thinking now is that they weren't referring to people who came out of nowhere. They were really referring to actual JWs who took advantage of the closeness and trust of the membership and made out quite well for a long time. It was probably a mix of trying to warn people to be mindful, but also not wanting to point fingers because they knew no one would believe it, and would more likely be suspicious of anyone who would accuse a brother of being a cheat.