When I try to short a stock, the transaction either goes through or I get a message saying the stock is not available for shorting. I don't see any interest being charged after shorting (except deduction for dividends). I called my broker and am told since there is enough collateral in the account you are not being charged any margin interest.

When you guys say that to short such and such stock an interest of 20% is being charged etc.. what does it mean?