If you have any luck at all, the Runescape Black Market, which can be found here: Click
Most likely though, people won't be urged to buy private server gold, as it is a private server, and not really worth anything.
It's not RuneScape gold, so it probably wont qualify under a RuneScape section, so I would say your best bet is in the MMO Item/Gold sales section in the 'Other game markets' sub-forum. Click here to go to the MMO Item/Gold sales section.
The owner sells 100M for about $4\10M He opens it and in two days or so gets like $500 donated. I'm selling for $2\10M so I hope to sell out pretty quickly. I will try the RSBM