"If you show up with your own trailer, then by statute, he must charge you his state's sales tax."
I've been involved in wholesale/retail sales/distribution since 1988. I've always known this to be the case, certainly in Michigan.
I just don't get how so many folks can simply cross state line and avoid sales tax. If the consumer has entered the retail establishment and taken possession of the product, it is not an interstate sale. Based on my limited knowledge, these retailers are responsible to pay sales tax to the state they're in based on the the final sale amount, whether they've charged tax or not. To me, that's a gamble I wouldn't dare take with my business. Sure, we might be a little lax in collecting resale ID's from customers, but I'd never let a taxable entity enter my store for an item without paying tax. By their taking delivery, without a resale certificate or some other tax exempt certificate, they are taxable.
Granted, an auditor would really have to be digging to determine whether the item was actually shipped or not.
So, in the case of these folks crossing state line and picking up their own tractor without paying tax (assuming both states have sales and use taxes) both the retailer and the consumer would be in violation of a tax code.
From a business standpoint, we're responsible for paying sales tax on taxable sales, less bad debt. There's a little math involved in backing out the amount of the transaction that would be considered tax, but bottom line is that the business is responsible to the state for the amount of in-state taxable sales, period. If you walk out of my store having paid 1 crisp dollar for a widget, the state will consider the sale amount as .94 for me and .06 for them, whether the guy walking out with the widget is aware of it or not. Same thing applies for a tractor.
Sorry to beat this up, but from a small businessmans perspective, that's a lot of risk for a business to take. (Having interstate consumers take delivery of product direct from the store.)