I know of a few ways not yet mentioned:
1) The Wall of Iron spell
a) Iron is a trade good, valued by weight (1 sp/pound), which means it's usable as cash.
b) The Wall of Iron spell makes a very large amount of iron, all at once (you'll need to look up the density of iron, and do some math to figure your profit margin)
2) The Fabricate Spell + ranks in a Craft skill
a) Materials required for Fabricate are the materials needed to craft the object in question - which is 1/3rd the market price of the item.
b) You can generally sell stuff for 1/2 market price.
c) a+b= 1/6th market price profit off anything you can reliably Fabricate, with a few rounds' casting (so, say, Masterwork Fullplate - Craft DC 20 - nets you 275 gp for a 5th level spell slot).
3) Lesser Planar Binding/Ally, Torches, and the Lantern Archon
a) A Lantern Archon is easy to Call by way of Lesser Planar Binding or Lesser Planar Ally
b) A Lantern Archon gets Continual Flame as a spell-like ability usable at-will
c) An Everburning Torch is a 110 gp item weighing in at 1 pound, and per the description:
SRD said:
Everburning Torch: This otherwise normal torch has a continual flame spell cast upon it. An everburning torch clearly illuminates a 20-foot radius and provides shadowy illumination out to a 40-foot radius.
d) regular torches cost 1 cp each.
e) If you purchase a hundred torches, Lesser Planar Bind/Ally a Lantern Archon, and have it spend ten minutes using it's at-will ability, you've got 100 items worth 110 gp each, which you can then sell for half (5,500 gp), having spent 1 gp (plus the requirements for the spell, which vary based on which you use). If you purchase a thousand torches for 10 gp, and have the Lantern Archon work for 1,000 rounds (1 hour 40 minutes), you've got 1,000 items worth 110 gp each, and you can sell them for half (55,000 gp). If you purchase 10,000 torches (100 gp), and have the Lantern Archon work for 10,000 rounds (16 hours, 40 minutes), you've got 10,000 items worth 110 gp each which you can sell for half (550,000 gp). Do note that the biggest bottleneck here is how long you can Bind/Ally the Lantern Archon - generally 1 day per caster level. As a 9th level caster (do note: Lantern Archons need neither rest nor food) you can get nine days of service, which is 129,600 rounds. If you spend 1,296 gp on torches (so that the archon has one torch per round to apply Continual Flame), you can sell those 129,600 Everburning Torches for half (7,128,000 gp).
4) Binding/Gating in Efreeti
a) Efreeti have the ability to grant 3 Wishes to a mortal, once per day.
b) Efreeti's are in range of Planar Binding, Planar Ally, or Gate.
c) The safe list of Wish includes creating a mundane item worth 25,000 gp, or a magical item (for an increased XP cost... although technically as a spell like ability, the XP cost doesn't apply).
d) You do the math.
Mind you, if you try to break the game this way, the DM will find a way to make it hurt (you're flooding the market on iron - it suddenly becomes worthless because everyone has all the iron they could want; you buy up all the torches in the land and sell an absurd number of Everburning Torches, the price skyrockets on normal torches because there is a huge demand but production is the same, while the cost of an Everburning Torch falls into the toilet because there is an absurd supply while the demand is the same; the price of mundane goods starts to fall, and you get attacked by a bounty hunter who was hired by all those crafters you put out of business, or all those people you Charmed/Dominate into giving away all their hard-earned wealth; the LE Efreeti twist your wishes and/or follow their racial writeup of a hatred of servitude and send bounty hunters after you (paid by way of Wishes); and so on). Thus, the best method is item 1 from StreamOfTheSky - kill things and take their stuff - as it's standard D&D practice.