Thanks for all your comments and suggestions.
I've decided to give it a go and have today set up the Buy & Sell Trade Cars Sub Forum.
If you want access to the site you need to simply email james@blackballmedia.co.uk with a copy of your current motor trade insurance certificate.
This, I feel, is the easiest way to prove trade status. VAT numbers are impossible to check - Google it and you'll find that anyone can claim to have a number but there's no database online to check them against.
Also, some people mentioned, and rightly so, that not everyone will reach the VAT threshold.
So I'm going to stick to the above for now. I'll be closely monitoring it and seeing how it goes, but there's no harm in giving it a go.
If you want access email me a copy of motor trade insurance certificate and I'll give you access. Let's see how we get on from there...