Yes, we are providing the liquidity for margin trading right now and looking at the possibility of allowing Kraken clients to loan to other clients. Let me know if there are any other questions about margin trading. Unfortunately bitcointalk was down when we rolled this out, which may have prevented some of you from posting here with your questions.
A suggestion how to design a user financed liquidity pool:
(1) Let every user donate their funds to the liquidity pool
(2) Calculate the return of the liquidity pool per hour
(3) Pay each user a share of the profits according to their liquidity share
Example:
The liquidity pool is 1,000BTC and has earned a profit of 0.01BTC in the last hour (0,001%). A user has invested 10BTC (1% of the pool) and thus receives after an hour 0,01BTC*0,01 = 0,0001BTC as a return. If he had only invested half an hour he receives only half of it (0,01BTC*0,01*0,5=0,00005BTC).
This procedure has five main advantages over the lending feature at bitfinex:
(1) There is no need for another market where users have to place orders
(2) Less risky for the lender: Each investment is guaranteed a return, instead of altering between no return and a higher return (averge should be about the same)
(3) Less risky for the borrower: The cost of borrowing stays constant according to the rules of the liquidity pool
(4) Users can exactly determine how long they lend money and change their mind at any given moment
(5) Autoreinvestments are easy
There are one challenge and one disadvantage to this kind of user financed liquidity pool.
Challenge: kraken needs to fill the liquidity pool in case a larger deinvestment of a user or multiple usesers leads to a temporary liquidity gap.
Disadvantage: Lenders are not able to generate their own conditions but have to take what the pool offers as returns.
I do not think that it is a huge problem for kraken to provide liquidity in case of temporary shortages. And the disadvantage of fixed returns for lenders could be overcome by offering multiple liquidity pools with different rates for the borrowers. For example, there could be a cheap pool which will be used nearly always and other pools of liquidity, which charge more interest and are thus used less often, for example only in volatile times. Users can then have the option in which liquidity pool they invest.