In the US this matters more by state than on a federal level.
Remember though you need to be aware of laws for each state you ship to. While this is next to impossible, you need to try to be aware of them as when you ship acrossed state lines the other state can prosecute you, plus it opens you up to federal issues as well.
Most states however require at minimum some form of sales ID/certificate, most states also require you to collect sales tax on sales made inside your state, some local communities and counties will require you to collect tax as well.
Best bet would be to consult your local state commerce department. An accountant would also be highly recommended.