Amazon and ebay taxes?

I'm visiting USA this summer, I'm staying in Philadelphia. I wanted to buy Nikon camera. I checked ebay and amazon stores and i found good prices there but I don't if the mentioned price is the final one or I have to add other taxes. And if there's extra tax how can I calculate it.

You are talking about two different sites, ebay does not actually sell you anything, anything you buy is from the person who has advertised it on ebay and any sales tax will depend on that person's status--if they are a registered retail business in the state you buy it they will charge you sales tax
Amazon is slightly different and due to court cases in recent years, more and more of Amazon sales will incur sales tax if they have a nexus in the state you are buying the item
sales tax will depend on the rate of the state where you buy it

Would NOT be normal for the mentioned price of an item to include the sales and use tax at that time with out telling you that the taxes have been included at that time.
Hope that you find the above enclosed information useful. 06/23/2014