You are likely to need a suitably sized van for collecting new stock, delivering items locally and taking things to and from auctions, fairs and sales. The size of van you'll need depends on the sorts of things you intend to buy and sell - it could range from something like a Transit-sized van up to a medium-sized lorry.
A heavy duty roof rack and a large trailer could be useful for carrying things.
The older the vehicle the less it will cost to buy, but very old vehicles may not be economical to run and may project the wrong image for your business. An alternative to purchasing a new or nearly new vehicle outright would be to lease one.
Make sure that your vehicle insurance company knows that the vehicle will be driven for business purposes, possibly by several different drivers. If you need goods in transit cover, tell them about the sorts of things you'll be carrying and their value. You should consider having extra security installed in the vehicle if you're regularly going to carry valuable items.
If you're planning to get involved in demolition and dismantling work then you may need your own specialist plant and vehicles. Some of these may be road legal, but off-road only vehicles will need suitable transporters to deliver them to sites. An alternative to buying specialist demolition vehicles would be to hire or lease whatever you need as and when you need it.