"The seller is responsible for the sales tax, not the shop."
Not if the seller doesn't have nexus in the purchasers state. For most of these transactions, the purchaser should be self-assessing and paying USE tax (it is included on your state tax return, if your state has a sales tax).
Whether sales tax was paid on the original purchase or not doesn't matter. The sale of used items is most certainly subject to sales tax, the same way that the sales of new items are.
There are three likely scenarios that are happening:
1) The local dealer is ignorant of the sales tax laws, and doesn't really know what he is doing.
2) The local dealer is scamming the customer and the state by charging sales tax and not remitting it. A call to the state tax agency may clear it up.
3) The state is getting more agressive about collecting sales tax revenue, and since it would be very difficult to audit individuals for out of state purchases, but very easy to audit a business, they may have simply placed the burden for the collection of tax on the easily auditble entity that is located in their state.
Scott