Nevada charges what, 7 1/2% sales tax? Las Vegas (or Clark County, if that's where the South Point is) may also require a business license for sales, if only for a day or two. The dealers who set up shop at the bourse DO report their sales. Exchanging money for chips by an unlicensed individual may be against the law and also subject the club (as the host for the event) to legal issues.

Having said that, I have in the past been approached by both individuals AND dealers, offering to buy a chip or two (usually new issues). In some cases, where a dealer has some chips I might be interested in, a trade can be worked out. Selling for cash, even outside in the hall or in the (FLUSHHHH!) bathroom can still be deemed illegal, if some sort of policeman or Sales Tax Official is watching.