You've got too many pronouns without antecedents in your post. The merchants are not liable to collect sales tax, but customers may be liable to pay it under their state law. This is called a use tax: http://en.wikipedia.org/wiki/Use_tax

Exact details vary from jurisdiction to jurisdiction... and in fact that's the problem this bill is intended to address. A federal framework is necessary to avoid the problem of being required to track an arbitrary number of non-Federal frameworks.