Ian - I'm thinking the problem would be when a landscaper buys plant material at wholesale price (pays PST on that) then sells it at a marked-up, or retail price to the customer. In effect the government would be shortchanged on the PST amount due for the difference of the wholesale and retail prices. Know what I mean?
So if I bought a $75 tree for $50 (wholesale) and paid PST at the nursery...then charged the customer $75 plus labor time and GST only...then there'd be PST on $25 the gov't didn't get.
I don't know. I'm gonna check Revenue Canada's website see if I can get an answer.