You need to narrow your quoting window.  You really should automate your trades on the exchanges too.

With the wild swings in the market, you really should be quoting live rates, or use a 15 minute window.  Maybe an hour, tops.  If your quoted price is different from the live prices on the exchanges, people will use you for arbitrage.  Since your operation is a way to go from bitcoins (liquid) to physical goods (not liquid), you won't have big operators cycling through you over and over again, but people that want to use your service will do it when it favors them, and wait when it doesn't.  Not absolutely, of course, but there will be a bias, and that bias is already big enough to make your site unprofitable.

Automated trading will limit your exchange losses.  Just sell them the instant that you get them, for the best price that you can get.  The exchanges have APIs that make this pretty easy.

Resist the temptation to speculate with them.  Either you don't know what you are doing and will take unnecessary losses, or you do know what you are doing and really should keep that activity separate.