I was thinking of flying into the usa, renting a u haul and buying a whole bunch stuff that I need sofas tvs etc in portland driving to vancouver bc, when moving there.

could that work

All the new stuff would be taxable at the border so you'd have to know the prices very well to be sure to cover the cost of the u-haul, especially considering the drop charge.

but when you move don't you get a tax free amount

In general, you can bring things you own without them being taxable but you can't pick up new ones on the way.

http://cbsa-asfc.gc.ca/publications/...f5113-eng.html

Ownership, possession and use requirements

To import goods duty- and tax-free, Settlers must have owned, possessed and used the goods prior to their arrival in Canada and Former Residents must have owned, possessed and used the goods for at least six months before returning to resume residence.

It is important that you meet these three requirements. For example, if you owned and possessed the goods without using them, the goods will be subject to duty and taxes. Please note that leased goods are subject to duty and taxes because the CBSA does not consider that you own them. If you have bills of sale and registration documents, they can help you prove that you meet these requirements.

Judging by the TV programme, CBSA staff will have a party when you show up at the border crossing!