They do definitely talk to each other and even the top level comics will give jokes to others when they think it would suit them better. I do think selling jokes is less prevelant but it still happens. I do know when it comes to televised roasts most comics get help either paid or just from friends.
There is a rift between Jay Mohr and Bert Kreischer that still exists as far as I know over freely given material. Bert used to open for Jay. He had a story that happened to him with Tracy Morgan. He gave it to Jay because it didn’t really work on stage unless you could do a Tracy Morgan impression (and Jay probably does the best). But one time Bert told the story in an interview and Jay got attacked by fans for being a joke thief. Even after Bert went on air to explain it he still got attacked for it.