Sales Tax rules can be complicated because it depends on where you are and who you ship to. It also depends if you have a physical presence (store) in a state that charges sale tax as well. If an online retailer has a physical presence in a particular state, such as business offices or a warehouse, it must collect sales tax from customers in that state. If a business does not have a physical presence in a state, it is not required to collect sales tax for sales from customers in that state. For New York State requirements, check out this free, downloadable guide (.pdf file) published by the New York State Tax Department.
Thanks to Nance for the information.