Businesses should collect sales tax only if they have nexus in a state. Sales tax nexus could be:
A physical presence (office, employee, storage or warehouse, inventory) in that state.
Marketing or sales affiliates that refer customers to your business.
Web cookies or click-through ads used on in-state websites.
A large volume or number of sales to in-state customers, a.k.a. economic nexus. (See next question!)
Each state has its own rules about nexus and when you’re liable for sales tax, so it’s important to check with local policies. Once you have a nexus, you should register for sales tax there and start collecting it on sales to customers within that state.