The floor is opened. Hmmm.
The Idea is great but your concept is crowded with too many solutions. I really don't know how you plan to go about getting involved in all the areas you have mentioned, but who knows you might be able to push around it.
at the buyer need to do is to call me. direct and locate the type of car he/she wants from the Website and i tell him the price, if he/she needs to see the car, i will take him/her to the Company that sell the car, then he/she will see the car.. and if intrested.. he/she will buy the car.
Do you have the time and the funds to do that?
My suggestion is narrow down everything into a very simple precise solution. Concentrate more on the frontend transactions rather than the physical backend transactions.
Let your website be more of the search and be informed, rather than search, choose and I would give you a tour, because I am the agent. You can get the datas rather from your clients and then forward the information to your merchant with your charges. So I would encourage you to get your clients from the website and forward them to the seller to follow up. Saves you time, resources and money don't you think

Your website can have all the information required in a catalog form with features such as the ability to book for test drives, enquirers, picture display of the product etc.
Now 10% on every car, I wouldn't agree to that if I am a car dealer

Honestly, I am very generous when it comes to giving out portions but this just wouldn't work, The question is why should I use your solution? How am I ensured that every second spent with you giving you all the information on the car is worth it? As in respect to just ignoring you and sticking to my other already used working conventional way of marketing that product. You do have to consider all this.
Anything .com especially when it comes to the Nigerian market, requires that you really convince the merchant that would be involved, that it is really worth it. But the moment you start mentioning figures when solutions haven't even been implemented, It would scare them away.
Go for something like a test marketing concept, give it for free, establish a reputation and later charge for it. Which means you need capital, of course you do, its a business

. An example might be First 2 car sales in one month is free, who knows you might be able to sell first 2 cars in 3 days

And to end it, I would say , you should get more accurate informations from the dealer, in regards to what you plan to do. A survey on what they wouldn't mind getting involved in would be great. All the best.