Your best bet mate is to contact the lovely people at the Tax office and they'll send you all the information out.

They'll ask you what you plan to do and keep a record of that iirc.

It may be worth opening a business account with your bank so you can get a cheque book and try and blag trade discounts at places?

I do the occassional one or two from home and use the Autotrader. You get the option of private or trade advert. The trade one is a bit cheaper I think and you don't have to put any trade details in. In the online form for each car it asks for a business name for the advert but I normally just put a full stop in the box and have never had a problem.

If you plan to use auctions, you will need to register with them before hand to be able to get the full registration documents. That's where the insurance and chequebook would come in handy but I'm not sure what else they'd need? Maybe print some invoices off with your address on too?

In fairness, the old boy has pretty much had everything set up for so long I'm only really going on common sense here... but I imagine searches and checks will be more thorough if you're applying for a credit line!