What Trader said, basically. It's not how much gold you have that matters, but how much gold others have which sets the prices, so aim to get the things even veteran players still need or simply desire and sell them on. Things such as imbuing reagents (the peerless spawns mentioned) but also things which are new or relatively rare; ET mentions the fishing scrolls as one thing a peaceful character could get, but right now the Charybdis items would fetch a fortune too as hardly anyone has seen him, and they don't appear to drop often.
It's not theory crafting either; I played from 2001 to 2003, and then took a long break. I came back to the game about a year and a half ago (2010), with literally nothing but my characters, some of which were outright naked as I'd given away everything. However once I found that there were plenty of housing spaces free, it was a question of sitting down and researching what people wanted. I made my first fortune of about 150 - 200m in 3 months mostly selling valorite ingots, gems and crystalline blackrock from mining. However, the largest single item sold was an EM Event one which proved to be exceptionally desirable (Firemans' Helmet)... Unless I get a lot of duplicates I don't tend to sell now as I'm more than comfortable, but even the most common and given away event item would prove to be worth about 1 to 5m though for rares collectors.
And if you don't want to get involved with bartering and all the hassle of being a trader, you can just wail on Tsuki wolves for an hour a day for silly amounts of pure gold. And unravel the items they drop; roughly it seems to be 1m for 10k residue and 2k essences on Europa. Get yourself a Luna vendor if possible and just leave your items on there... which reminds me, mine closed yesterday, I need a new one alas!