"Risks
Lending to margin traders on Poloniex carries three main risks for the lender.
Exchange insolvency: The cryptocurrency space has been plagued with exchange hacks and failures, such as the now infamous Mt. Gox. If an exchange becomes insolvent or otherwise loses customer funds, there is a high probability of a substantial or total loss for the lender.
Exchange outages: Exchange outages, and periods of high volatility are highly correlated because the increased trading volume caused by volatility taxes the exchange’s servers. A temporary exchange outage during a period of high volatility could lead to margin traders losing all of the collateral in their accounts as well as some of their borrowed funds. In this situation, the trader would be unable to repay the entirety of their margin loan.
Currency Risk: When funds are on loan to margin traders, they are not available to be exchanged until the margin loan has matured. A price ‘crash’ on an asset that you are lending, will frequently be in the favor of the borrower. Because the borrower will be making money from the crashing price, they will not want to close the loan, and you as the lender could be left holding the bag."
From:
http://blog.poloniexlendingbot.com/margin-lending-basicsAbout the fees... EDIT: you pay them when they get filled.