I always find this very interesting with betfair.

Basically the minimum back bet allowed by T+Cs is $5.
The minimum lay bet though is also $5. But in order to lay a $100 selection you are going to really need $500 in the account.

They don't seem to understand that really it should be risked amount rather than the amount showing on betfair. A $5.00 lay on a $100 horse should allow you to do 5c with minimum liability of $5.00. Your risking the exact same amount as backing for $5.00

But that is not betfairs point of view. So if you want to lay long shots you need a very large bank to be within the T+C's.