Thanks for the explanation, it's a very interesting system, economy wise ... more like options than a pure loan system (see below)
with a deposit of 100NMC for each borrowed NMC
You mean "for each borrowed BTC".
The interest calculation is not right. It should be based on compound interest calculations, not anual / 12 * months.
E.g. in your example the interest should be 1.05^(3/12)-1 = 0.0123, which in BTC is 0.123 BTC.
I guess you can use your method of interest calculation since the difference is not huge, but the "correct" way to do it is compound interest.
The deposit ... should be higher than the current market price, otherwise I would borrow the coins, sell them and immediately close the loan
This explains why the deposit should not be lower than the current market price, but not why it has to be higher. The reason for it to be higher is market volatility - if I loan, wait 3 months, and pay back, and the BTC/NMC rates haven't changed, no sides would have lost - however, if NMC is worth a lot less BTC after 3 months, then the lender would have an incentive not to pay back the loan.
Actually, when I think about it, a better way to think about it is as a standard loan, combined with a CALL option. The lender buys, for a price equal to the interest, the option to buy the deposit back at the price of the loan, for the duration of the loan.
In this example, the lender simultaneously buys, for the price of 0.125 BTC:
- A zero interest loan of 10 BTC for 3 months, with a deposit of 1000 NMC
- The option to buy 10 BTC, to be paid in 3 months, for the next 3 months, for 1000 NMC, where the 10 BTC "are paid back" at the end of the period (in effect negating your 10 BTC debt)
There is a slight difference in attitude between describing this system as a loan system or option system. With pure loaning, default on a loan is considered to be "a bad thing", and will prohibit people in the future from loaning to the individual who defaulted. With the system you described, it's perfectly moral not to pay back a loan, if the BTC/NMC rates make it more worthwhile to keep the 10 BTC, rather than get the 1000 NMC back. The lender should factor this risk into the price, and be aware that he's actually selling options, not just a pure loan.