Stay Liquid
You never want all your money tied up in one item. You want to have at least half your money in cash (preferably more), in case a good deal comes up. The exception is if you are getting good deals, or really easy to sell items.
Generally I like to be liquid and have lots of cash, rather than lots of items. It's risky to hold items, cause yes, the price could go way up, but then again, it could go way down.
I have bought many items and then sold them at a profit, only to buy it even higher later on. Like I bought a dark battle duck for 700k, sold it for 750k, only to buy them later at 900k, and sell it at 950k. Wouldn't it have been smarter to just hold it and sell the 700k one for 950k later on? Yes. But I didn't know it was going to go up, and it could just as easily have dropped to 400k. Plus in those few weeks (it took weeks for it to go up that much), I used that 750k to make trades and win auctions, which made way more than the 200k I would've made by just "holding" an item.
I buy and sell within 24 hours, within 6 hours of possible. That way rapid price changes don't affect me too much. And I have many times bought items for a higher price than the price I sold them for just a few days earlier, (like the dark battle duck example). However, I don't mind.
Like I have bought map pieces for 21k, sold them for 23k one day. I would then buy the exact same piece for 24.5k a few days later, only to sell it later for 26k. The following week, the price dropped back down to 24k. I have seen the value of many items rise over time, but I have also seen the value of many items plummet, so I don't take the chance of holding items for more than a day or two.
In the few months I played in April-June 2003, I have seen codestones rise from about 4,300 to about 4,700 and drop to 4,000. I have seen the Faerie Paint Brush Plushie go from 110k to 80k, up to 90k. I have seen the Jhuidah Faerie Doll drop from 150k to 100k, but I have seen battle ducks go from 170k to 250k, and Honey potions go from 380k, to 350k, back up to 380k. I have seen the two dubloon drop from 3,800 to 3,500, while the ten dubloon fluctuated from 18,000 to 16,000 and inbetween on roller coaster price swings. I have seen the hidden tower screw things up, and see the Baby Paint Brush from a 330k, to a 380k, to a 450k, to 480k, to 450k, while the Faerie Paint Brush went from 330k, to 350k, to 330k, to 200k, then 190k.
The way to avoid all the risk of price changes is to just buy and sell fast, then it can't hurt you.