Yes, the commission you charge the client would be a taxable supply.
Also, considering you are facilitating the transactions, I assume that if your third party seller is VAT registered, you would charge the purchaser VAT on their behalf? And only pass the VAT back to the third party?
The third party would include VAT in their pricing on my site, so the total price paid would include the VAT and would be passed on to the third party in full. The VAT for this supply would not be any of my concern as the contract is between the customer and this third party seller, so it is up to the seller how he accounts for VAT.
You are most probably wrong there, unless the UK VAT rules are completely different from the base EU ones. I run a company from within Bulgaria as well, and while the invoices sent outside EU is 0% VAT based, they do count towards the company VAT threshold (in the regards to if the company needs to be VAT registered). This is also how it works here in Norway for any invoices I send to foreign clients.
If I supply electronically supplied services to customers 'enjoying' those services outside the EU, they are not even zero rated, they are out of scope, they are not even taxable supplies - there is a big difference between 0% rated and 'out of scope' - out of scope means it doesn't even exist as far as VAT accountability is concerned. [Check this PDF for UK businesses, or [URL="http://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPortalWebApp.portal?nfpb=true&pageLabel=pageVAT_ShowContent&id=HMCE_CL_001730&propertyType=document"]this](http://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPortalWebApp.portal?nfpb=true&pageLabel=pageVAT_ShowContent&id=HMCE_CL_001727&propertyType=document) for EU businesses. I am not aware of the rules in Norway, but my understanding is that UK and EU VAT laws are the same so these should apply to Bulgaria.
On a side note, what disadvantages are it you guys see with being VAT registered?
As from a company side point I see only benefits with this, and all the companies I'm running I have elected to make them VAT registered from the start (since I knew we would pass the threshold withing the first few months anyway). This has really saved the companies money on the initial purchases for hardware/services etc.
We sell digital goods mainly to consumers, 25% of whom are in the EU. These customers would see a 20% increase in our product prices if we were VAT registered, so if we can avoid it, it's definitely beneficial compared to the saving we'd make in reclaiming a comparatively small amount of VAT each year from hardware or services we purchase in the course of running the business.