Hi Dean,
I can imagine that the rules and regulations vary per country, but in general I would think that responsibility for Export and compliance lies with the supplier of the goods. In the case you describe, you are acting (only) as a sales agent, brokering a supply from the delivering plant to the customer.
If the customer is local, then according to the Incoterms, you might have responsibility for the Import. And you might also want to carry out SPL checks on your customer. But the export declaration, and the provision of any export licences, should be the responsibilty of the supplying plant. Hopefully you can also persuade them to use GTS!
Regards,
Dave