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.

The purchaser has always been responsible for the sales tax. However, in America, almost all taxing jurisdictions implement a withholding charge on the merchant, so that the merchant collects the sales tax owed by the purchaser on the purchaser's behalf.

Quill v. North Dakota, the SCOTUS case to which you refer, was about a sale-order catalog's obligation to withhold the sales tax for purchases by out-of-state state customers (in the case, customers in North Dakota). SCOTUS ruled that North Dakota did not have the jurisdiction to impose a withholding obligation on Quill because it did not do sufficient business in the state to have a "presence". However, after the case in various speeches, several of the deciding justices noted that if Quill had had a greater volume of business (on the order of an in-state retailer), the outcome would probably have been different.