So to summarise, I play by these general rules:
1. Don't salvage drops unless you need the crafting materials...it's worth more to sell to the merchant rather than salvage and sell.
2. Research on Guildwiki what collectors items are required for max damage weapons and max armor for, essentially free, armor and weapons.
3. Find a nice farming spot. An often popular one can be Kirin/Undergrowths in Pongmei Valley giving ~1-1.5k/run
4. Find a solo farming build for green weapons. Personally when I get low on cash I wander around and annoy Mr. Scar Eater.
5. If you choose to farm green weapons farm ones which are easily sellable. For instance, although there must be thousands of them out there the Scar Eater still sells fast...other easily farmable weaponssuch as Darkroot's Daggers take forever to sell (from personal experience).
6. Hold back any nice weapons for your heroes so you don't need to buy some specially...My ele runs chests so I tend to keep any crappy max-damage golds/purples for the heroes...
7. Find a service people want. I recently made my Dervish D/A and run to Droks...being a runner can be a popular way of making money.
8. Find one or two usually favourable characters, from a farming point of view, and try to get them as far through each campaign as possible so that if a new farming build comes out you have no problem buying up new skills or capping elites that are further through a game. Popular ones include Warriors, Elementalists, and unfortunately Assassins...Monks are always a classic choice from a farming perspective.
9. Plan ahead. If have a Warrior and you think you may want to start an Elementalist in the near future and you get a nice staff drop for you...keep it. This will save you finding collectors items or buying one later on in the game. This can go too far though...personally I hoard far too much
10. Go through mission and do side-quests...you'll be amazed at how fast the cash rewards from non-prophecies campaigns can mount up.
11. Know the value of your drops. Don't sell the merchant a potentially valuable drop. You can always double check by going to the
Price Check forum and starting a thread to ask about your weapon.
12. Learn common trader's tricks and scams. GuildWiki has an extensive section on this
here. Check
and double check
both what they have in their trade window
and how much money you have put up in yours.
13. Watch out for freebies. Occaisonally you get some nice players giving away free items. I do from time to time.
14. Plan your builds. If you know what builds you are likely to want to play you will not end up buying skills you don't need which can get costly at 1k per skill.
Hope this helped a bit

Good luck.