"Who gave us that legal advise? Did they have a conflict of interest to disclose?"

Same person as the last time you asked - Eric Rosenblum - who personally explained the position to you at one of the annual meetings. Don't tell me you forgot. If you want to argue about that, take it up with him directly.

"We have third-party for-profit companies/people doing buying and selling at our conventions. Is that OK and legal?"

The club effectively acts as an agent for the South Point in renting floor space and then sub-letting that to vendors. It does not endorse any specific vendors who are buying/selling within the rules and laws of the South Point Hotel and Nevada State. That is a whole different board game to allowing commercial activity to take place on a forum 100% owned and therefore endorsed by the Club.

"Is it OK/legal for members of the Board of Directors to sell for personal gain at conventions? Is there a double standard here?"

No, for the reason stated above.
Maybe if more volunteers who were not chip sellers/dealers put themselves forward for election to BoD positions then the question would not arise. Seems that plenty of people have tons of spare time to repeatedly make negative/critical posts yet vanish into thin air when asked to actually physically volunteer.