If you live outside the US but the supplier is asking for a sales tax ID, the supplier will usually want to see the closest equivalent to a sales tax ID for your country. For example, if you live in Britain or the EU, you will need to be a VAT registered trader; in Australia and NZ, registered for GST. Alternatively, the wholesaler may want to see your business license/registration details.
You may just go ahead and communicate with them. FuturePets.com is a verified SaleHoo supplier and we have not received any serious complaints about them from other SaleHoo members. :)
Let me know if you need us to help getting in touch with this supplier.