Why didn't Tesla use his machine to make money before selling it to Angier?

Apart from the obvious explanation that this was just not considered in the story and wouldn't have fit there, there is evidence from the movie itself and its depiction of Nikola Tesla.

Tesla was afraid of the machine's power once he realized what he actually created there. Remember that when he gives it to Angier he also writes something similar to

The only advice for the machine's usage I can give to you is, destroy it!

So Tesla built that thing for him, but he also realized how much power this thing actually conveys and the potential danger this machine poses outweights Angier's possible use from it. I think he mentioned before that he thinks humanity might not be ready for all his ideas, and this machine is an instance of that.

One might ask then, why he still gave it to Angier instead of just destroying it right away or not finishing it at all. But I'd assume it is partly his pride in having achieved such a scientific breakthrough as well as his inclination to honor the contract and promise he gave Angier. In addition to that it might also support the common motif of the scientist who only realized the true danger of his invention after the fact and is reluctant to take full responsibility for it, just let Angier take the hard decision to destroy such an amazing technological achievement.

Last but not least, Tesla didn't seem too inclined in making money by dirty tricks instead of by genuine appreciation of his developments. He wasn't really the person who did all this to make a fast dollar, but for the progress of mankind and if he would take money, then because his products are well-received and appreciated, but not just by misusing them to produce "dirty" money. In this regard he might indeed be seen as a worse businessman compared to Edison, who didn't always play fair afterall. (And in turn this contrast between Tesla and Edison is also used in the movie to emphasize the rivaly between Borden and Angier.)

From the script:

TESLA (VO)

Alley has written you a thorough set of instruction. I add only one suggestion on using the machine - Destroy it. Drop it to the bottom of the deepest ocean.

Such a thing will only bring you misery.

It appears that Tesla is either frightened of the machine, or is pragmatically recognizing that the power such a machine offers is unlike anything in the world and could result in ruin for the owner or even more widely.

It is speculation, but perhaps Tesla realizes the temptation to make unlimited resources would become an end in itself - while he struggles to make a name for his technology in the world, perhaps he would rather an honest struggle than to follow the temptation of using the device itself.

I was always my (perhaps incorrect) assumption that Tesla didn't even know the machine worked. Remember all the hats out in the forest? I thought he didn't realize that things re-materialized elsewhere, so he wasn't ever going to step into it.

Working on that assumption, I guess once he discovered exactly what he did then he could have kept going back to the pile of transported money, throwing it back in the machine and then doubling the sum (and on and on...), but by then Angiers had already financed the machine so it really wasn't Tesla's anymore.

Whose to say that the fictional version of Tesla portrayed in The Prestige didn't duplicate large sums of cash prior to leaving Colorado? The movie doesn't show much of what Tesla was up to when Angier wasn't around.