Welcome to TBN. I'm a little closer to the OR/WA border - so the dilemma you have is VERY real to me. At the same time, because I live in a "border county", we have a little more experience with what the rules and regulations are.
The correct answer is that you are required to pay WA state sales tax on anything you buy. If you buy goods in Oregon, you are supposed to stop in at the tax office and pick up a form so you can report what you paid for the goods, how much tax you paid in the state you bought it in ($0 for Oregon, around 5.5% in Idaho, etc), and then send in a check for the difference to the state of Washington. The law allows the state, if they find out you 1) did purchase something out of state, and 2) did not pay WA sales tax on it, to send you a nasty letter and give you 30 days to pay the tax, or they will come after you for tax evasion. The reality is that about 95%+ of the purchases made in Oregon are not reported voluntarily, and about 95%+ of the ones not reported voluntarily are not reported at all. The people in the tax office will actually look at you like you have two heads if you do ask for the forms. An article in our local paper a few years back stated that most of the untaxed sales that are reported are those turned in by disgruntled neighbors or neighborhood "do-gooders" that see the owner pull up with a brand new plasma TV or refrigerator or whatever in the back of their truck.
It gets a little tricky when you look at things like tractors, vehicles, and other "big cost" items, etc. Anything that is delivered to you by a company that has multiple locations, including locations in the state being delivered to, will be required to report and charge the sales tax because you are in fact buying from a vendor in your home state. A good example would be buying a washer/dryer at Sears in Portland, but having it delivered to your house by Sears. They have stores in Washington, so their accounting has to report sales/deliveries in that state as well. But if you buy it from Sears in Portland and pick it up in Portland, you are now in the position where you are supposed to report it yourself. Depending on the vendor, even if they don't have stores in your home state, they technically supposed to charge and report the sales tax if they are making the delivery to that state. Some vendors simply don't pay attention to that requirement, but if caught they can be fined for it. So, usually, when you deal with a company in a "border" community, they will be aware of and honor the tax laws of the bordering state(s).
If you buy a car in Portland, they will get you when you go to register it. Pretty simple there.
If you finance something, the finacing will be reported to your home state because you are a resident of that state. In that situation, you are required to pay the taxes in order to do the financing through the dealer. If you were able to get your own individual financing without disclosing what you were buying with it (a home equity loan maybe) - then as long as it wasn't a bank transaction (i.e. hand them cash) it would be hard to trace.
So, to answer your direct question...the only way to easily avoid the sales tax by purchasing in Oregon would be to 1) buy from a single-location dealer, 2) pay for the item in real paper cash, 3) pick up the tractor at the dealer.
Of course, like ByronBob said, you will be losing out on good local dealer support (while required by the manufacturer to honor warranty items, your local dealer may not be too happy or enthusiastic about doing work on something he didn't sell). And, if you happen to be driving over the bridge into Washington and get stopped by the WSP (they do occasionally monitor it) and asked for any sort of papers on the machine, you may very well end up paying the taxes anyway. Besides, between the Kent area and the Eastern Washington (read: agricultural heaven), you've got a LOT of good dealers and brands available to choose from.
