Sorry but I am a smiley's lover
i will try to use them rearly.
And my friend i am not sure what a valid collateral is it something like escrow?
lol don't worry about the smileys, I was just making a joke.
collateral is some kind of assurance. the borrower will give the collateral to the lender/escrow. and if the borrower is not able to repay the loan the lender have the right to hold/sell the collateral. basically the collateral will be yours if they default.
you can find what kind of collateral is valid and what is not on this post. bitcointalk accounts can also be used as a collateral, but like I said, not everyone agrees with that way of thinking.
oh, and it is advised to use escrow for holding the collaterals.