OT(ish): Selling software downloads - sales tax?

[Graham Samuel]

I have been discussing the sale of a desktop product (developed with LC, obviously) with a colleague. We’re thinking of direct sales, rather than selling via an agent or an app store type of arrangement. He’s in the US and I’m in the UK, or at any rate the EU. 

I think that if I sell a low volume of product purely as a download with no physical fulfilment, I won’t have to charge sales tax anywhere in the European Union. But I am not sure about the US. It seems that some states charge sales tax on everything, while others (California, perhaps?) don’t charge if there is no physical fulfilment. I am not sure if the volume (or more accurately, the total value of sales per year) comes into it or not. It certainly does in Europe.

Some people on this list are selling apps, stacks etc in this way - can anyone explain how they see this? 

This may not be the right place to discuss the issue, but looking at the only forum topic which seems close - ‘Marketing Your Products’ - it seems pretty dead.

TIA

Graham