Ask YC: Taxes for selling user-generated content

Part of our startup's functionality is to offer the ability for users to create content and then sell it to other users through our service. We handle the payment processing and issue cheques at the end of the month (minus our small fee). We're doing tax research right now and wanted to know others' experiences with this type of thing.

We're in Canada, and we've got the Canadian stuff mostly figured out, but we're trying to figure out if and what taxes we need to deal with when a US customer sells to another US customer, for example (or any combination of US, Canadian or international customers).

Any advice, experiences or resources anyone might have would be fantastic!