collateral for lending out bitcoins are some told of altcoins, but be carefull that they are saleable altcoins because he defults it and when you go for sale in market then their should be seller for that. you can have bitcointalk account as collateral but for that you should be knowing some process like get signed message from the address mentioned in that bitcointalk account which he/she is going to give you in collateral if you have any doubt you can use escrow service in this process so that in future no one can come and scam you.