To answer your initial post, and like someone else has said, the sales are your best bet. AT, Gumtree, eBay and then work backwards. Make sure you factor in enough when it comes to prep. What kind of cars will you sell, anything and everything or diesels or estates or cheap sports cars or family Ford's or hot hatches?
This trade, and I'll probably be vilified for saying it, is fairly closed. And to be honest, rightly so. Everyone assumes they've never seen a poor car dealer, but what one man classes as a living another could piss away on a day out at the races.
In my experience, it took me nearly 6 years and about £15k in losses working part time and building contacts until I realised one thing; this isn't built for the weekend dealer. Between actually dealing with potential customers (worst and best part of the job), you've got to source the right stock at the right price. It's changed from the 70s where auctions were practically the only place to go for dealers, we're now in the information age and if you look hard enough there are options. Having said that, I'd hazard a guess and say all dealers still buy at least some proportion of stock from the block as it's easy. If book prices are met or exceeded and you feel they're too steep, don't buy anything, simple.
I used to download catalogues, research prices and then print them out and write next to the lot number what I'd be willing to pay and stick to it, until I learnt the trade and built confidence. It also makes it easier when vendors and auctioneers know who you are, as I've seen them taking bids off the wall and driving prices up a grand or even more.
Like anything worth doing, it won't come easy. But think about a few main points
Where will you sell from? If a premises, where are you and what's the going rent. If you're going to sell from home are your neighbours ok or tossers? I'd be a tosser if someone was selling cars on the road I lived on.
What will you buy? Or at least try to buy?
Do you have a paint shop and mechanic you can trust?
You'll need a warranty company that pays out (Hardy har)
Insurance is a bloody killer
If you're working by yourself, how will you find time to source stock?
Have you got the capital to invest in stock? Like I said, after me personally losing a small fortune I then took a massive punt and invested in my first 7 cars and built from there.
Will you trade up, or deal in numbers?
Will you work off volume of sales or profit per unit?
Will you offer finance? If so, through who? You'll need a license as I'm sure you know.
You can be a "dealer" who (we all know one and I personally HATE them) who advertise as private sellers but are dealers. These guys give us a bad name and don't take their job seriously. They have sod all infrastructure and I'd bet don't pay any tax at all.
Or you can actually try and make it work, properly, build a brand and deal with the good and the bad (much more of the latter) but this will take a massive amount of time, money and even more effort.
Anyway, this post wasn't meant to be too long and it is so I do apologise, but think long and hard.
Rish