With that, I could technically use the details of that to act as ID for other things (I obviously won't as that's fraud), but the option is there for someone to abuse if they so wish.
Yes, we're aware of the possibility of identity theft and/or fraudulent documents. We have 3 trust levels: Bronze, Silver, and Gold. The Bronze level basically means untrusted. Our Silver level means we've done a basic ID check, which could be open to fraud as you say, so no collateral amounts should remain low like under $50-100. Our Gold level means we've done deeper checking into the user's background which is not easy to fake, but amounts should still remain low. For loan amounts of hundreds or thousands of dollars it should be certain the user is who they say they are, which is possible to verify, but takes more resources.
Looking at this. I'd say a good thing to do is offer (for a gold status) two options:
1. The borrower must provide collateral to the value of 120% of the loan (obviously)
2. A company name (lending to a company is great as you don't run into any licensing issues). If the company can't pay, you can always pressure the director by saying that they should be able to pay the debt - as they or the ceo can usually be held liable for the entire or part of the debt (in a lot of countries) so it's quite a helpful trick.
There's also a difference between people in different countries. $5 in one country might be a weeks worth of food (or even more) in some really poor country.
That's a very good point. In Venezuela right now $5 is about the average monthly salary a person has. However, I think that's all the more reason to try protecting one's credit rating. Say a Venezuelan built up their credit to be trusted with $30. For lenders in the US that's no big deal, but that's 6 months of income for them! They might be able to get a business idea started. It can't be that expensive to open a shop if most people earn $5 per month.
Alos, people might try working for getting a better credit score on your site just to try to get more.
That's exactly what I want to happen!

That's where the incentive to repay comes from.
Unless you say, someone can lend $5, the next time they come to your site, they can lend their original $5+the interest the paid once returning the $5.
It would be tricky to get the incentives right, but there may be a way to make it work. Thanks for the great input!
For this bit, my suggestion was probably something that would be a bit slow.
Maybe offering to double the original amount up to about $100 and then change to adding half each time might work quite well (and then increasing by a decreasing amount each time).