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?
not my site, but my account, i've made 20 btc with this account, so yes it's worth much more than a mere 1 btc
and no, altcoin ar enot a valid collateral they depreciate very easily, and a very unstable, i would not take any altcoin as a collateral not even litecoin
instead an hero account is still worth at least 0.5 even today
Hero or legendary account is a type of valid collateral considering that no many such accounts are existing at the moment. For the low level member accounts, altcoins are the best provided collateral. Most of lenders require the altcoins be listed at the major exchhanges and have high trading volume in case of liquidation. The high volatility may be a concern. The lenders could ask for 30%, 40% or more than the loan. In case the value of collateral drops down from the loan amount, lenders could set the rule asking to add more collateral.