Can you please help with our revenue model?

It's relatively simple if I understand correctly.

If the buyer wants to use the supplier they should pay you the deposit, which should be at least equal to your commission, through your site.
You simply keep the commission element and forward the rest if any.

The buyer has made a commitment, the supplier knows that they are unlikely to get burned and you receive payment in advance.

There are several Paypal type services out there who will supply you with a code to put a button on your website which will direct the buyer to a secure site where payment can be made securely direct to your chosen bank account.

I don't see any risk.

Interesting one. We have a partner called Service Magic whose model I think is to charge the supplier once they have a confirmed client. The user puts in a quote for whatever work/tradesman they want which is then supposed to be sent to 5 different suppliers.

Tradesman sign up for free, but pay an upfront fee once they can see there is a job request relevant to them.

The biggest problem they have is getting suppliers on board, and we tend to hear complaints form time to time about poor quality leads, or no replies from tradesmen when a user submits a job or quote.

Or maybe provide an escrow service if its staged payments? Keep the money in your account and take a small commission for each payment? Releasing payment when the customer is satisfied.

I should imagine you have deep pockets to get this off the ground and get enough suppliers on board, best of luck. Be great if you let us know how you're getting on!

I have over the years signed up to many of these types of sites, in my opinion they are misleading both customers and traders.
The problems are (1) misleading the public, by saying they check / vet the trader, the only checking that is ever done, is to make sure that the tradesmans credit card details are correct. (2) There are far too many of these types of sites around, so the consumer that uses these type of sites will generally register with 2 or 3 sites and get around 10 quotes, but the trader will think he's only quoting against 2 others. (3) Most traders that use these sites will not be registered for VAT and only work for cash. So if you operate / trade legitimately you cannot possibly compete in an unfair market. (4) From experience I find that most consumers that use these types of sites are time wasters, with maybe 1 in 20 chance of securing the work compared to 1 in 3 when a customer visits / calls direct.

I have to say I would be surprised if this business model would be very effective.

Some very good points have already been made about trust and the money issues, and for my part, although in principle it seems ok in practice I cannot see it evolving into a workable business arrangement.

You have already encountered problems signing up suppliers, I don't think that will change. Many have already signed up to this kind of service and found it to be a waste of their time. I can tell you now, I'm one of them. Even if it did work, with margins getting tighter, a 10% commission would take away any benefit of having a new customer. Suppliers could't add the commission to their prices to mainytain a decent profit as they would become uncompetitive.