The price changes so much over the course of a day, it's hard to know what to charge.

My suggestion is, think of what the buyer would do if they couldn't buy the coins from you and had to get them somewhere else.

Would they have to wire money to an exchange? How much would they spend on fees? How much is the convenience/time worth?

If they could get them from someone else locally, how much does that person charge?

Most people just use whatever price they are going for on Bitstamp or Coinbase, which is hard to argue with. For a deal that takes time, though, you might go with the 24h average.