the best collateral would be an account, it's easy to transfer the ownership and can be sold quite easily as well, but for 5 btc you need something like legendary with trust or hero with many trust
The best collateral would be an altcoin. I have seen plenty of loan requests using domains as a collateral and most of them are not accepted because the user is offering a shitty domain, if you can prove your site is worth 5btc or more then I'm sure someone will. Why do you need the 5 bitcoins in the first place?