There were many methods of selling gp.
1. Transferring the gold
- People found many methods of transferring gold (really obscure). A lot of people also sold guides on transferring gp.
2. Junk trading
- People bought junk items worth a lot. For example, broodoo shields were junk worth 30K+ each. The people who would sell the junk to the buyer would pay the buyer real $ for buying the junk.
3. Gold Farming
- This is still going on today. Basically, the GP seller logs onto the buyer's account and makes the gold by doing various tasks such as killing green dragons, picking flax, smelting steel bars, playing the Sorceress Garden minigame, etc.