You 'could' use your ebay (motors pro) Showroom for your website until you decide to take the plunge.
You'll need to buy a domain name of your choosing first from somewhere like easily.com or Godaddy etc. Then copy and paste the very long ebay showroom address from the address bar into your account you set up, and have this new company address (e.g www.mark101.com or something better obviously that you've chosen) forwarding to the ebay showroom address. So basically it forwards to the ebay showroom.
Hope that all makes sense, and I wouldn't pay Autotrader for anything if it was me.
Otherwise, Click Dealer are brilliant, but I'd wait a while if I was you. Take small steps for a few months is my advice. Build it slowly.