If its any older than 3 years it wont even touch a main dealers forecourt, that said if not a main dealer then they might look at the mark up on it.
They mostly go to auction, although sometimes they will ring round the local traders to see who offers the best price.
If your thinking of buying at auction, then do your homework first.
You may have VAT to pay on top of your purchase price and other fees.
Most large auction houses have a website, so you can see in advance whats going through.
One night will be Ex Fleet and Finance and another MOD or All one Firm such as Anlglian water, BT etc.
Buying from auction is not as bad as it used to be.
Best thing to do is go afew times to watch and see how things work, watch the money the cars go for make note of the price.
Compare those prices to Retail, taking into account a Good service, Tyres etc and work out what your saving.