We're Oregon ex-pats, now SD "residents", and SD makes it very easy to be a legal resident. Other than registration fees, sales tax is immaterial to a fulltimer - you pay whatever tax is appropriate wherever you buy it, for the most part. We save up our major expenses whenever possible and get them in non-sales-tax states. One more reason we still like Oregon a lot, now that we don't pay income tax there.
As far as internet purchases, those sites that collect tax usually do it based on where your order is delivered, not on where the billing address is. And which states get tax applied varies from company to company. Amazon probably collects tax for more states than anybody. Has to do with whether the company has a "business presence" in a particular state, and how the particular state defines a "business presence".
I agree with the other posters - since your plan is for 12-18 months of fulltiming and you've already decided on AZ, either leave everything legally in Oregon until you settle down, or go ahead and make the switch to AZ now.
And unless you have a big ticket house, there won't going to be any capital gains when you sell it, so there's no limit on when you have to roll the proceeds over into the next one. (NOT a valid legal opinion, just personal experience.)
al