That's my original question, because my customer claims I have to accept his certificate. While I decided to accept it on his $150 order, this question is still pending.
Is there a reason why you wouldn't accept it? I can't think of any reason not to.
However, the onus is on the seller to ensure it's a valid #.
I can only comment about Ontario, when a customer gives you a signed form THEY are the ones responsible to ensure that they are entitled to claim an exemption. As I mentioned previously one can be exempt without having a Vendors Permit No.
Companies that do not do ANY retail sales have no need nor any requirement to have a Vendors Permit but they can still purchase PST exempt by completing the form.
On the form, In the area for the permit no it says "Permit Number (If Applicable)".
It will all be ancient history for Ontario businesses come next July.

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
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