1. Go to https://bitcoinaverage.com/ and get the current price ($784.19 as of the time I'm typing this line)
2. Calculate the bitcoin amount of the USD amount you wanted to sell the item for (e.g. in my this thread, it is $1.20 = BTC0.00153024 as the price above)
3. Mark up it for 5-10% to a rounded amount (in that case, BTC0.0016). This is needed unfortunately due to the volatility of the market
4. Sell the product!
---OR---
1. Sell the item for '$X in bitcoins' and mention which site to reference the bitcoin price
2. After you found a buyer, go to that site and get the latest price at that moment
3. Calculate the equivalent BTC amount. No need (and don't!) mark up the price this time.
4. Success!
Unfortunately if the bitcoin price crashed more than 10% in a day (though unlikely but had actually happened), you will get a loss. You may want to keep the bitcoins as an investment for a hope that they will rise again.
Your calculation and idea is good but i think he can implement a software where it will calculate the price from the site and update it in every hour in his site so that nor he or the purchaser gets loss in the deal. If Op is going to deal the product sale manually then he can do it when ever a purchaser enquirers about the product and OP can give the current price according to the bitcoin market price