perhaps have a chart/graph on your site so they can clearly see the price changes.
I suspect anyone buying will know the price constantly changes... and if not they are stupid.
what if you sell for 590 and it goes to 650 ? will they give you an extra 60 a few days after they bought it. i think not :p
i found a very good solution for this .. thanks to Coinbase .. they have a service of real time converting between USD & BTC
thanks for your help guys