Know your rights
Wherever you buy from, consumer law is generally on your side. You can find detailed info at www.consumerdirect.gov.uk and www.tradingstandards.gov.uk/advice, but here's a quick summary of your rights. These apply when you buy as a consumer, not a business, from a commercial dealer.
Under the Sale of Goods Act and the various bits and bobs tacked onto it, anything you buy must be as described, of satisfactory quality and fit for its stated purpose. Your contract is with the supplier, not the manufacturer (though if you buy from Apple they're effectively the same), and you can only hold them to descriptions issued by them, not stuff you happened to read somewhere else.
The purpose the goods must be fit for is any purpose that was mentioned in the supplier's promotional bumf or that you mentioned when buying and they didn't say "No, it won't do that." It's well worth telling the seller what you plan to do with your new Mac, so this becomes part of the contract.
If the goods are unfit for their purpose, not as described or unsatisfactory, you can and should 'reject' them and ask for a full refund. These days, 'unsatisfactory' is interpreted quite broadly. Macs are sold as objects of desire, so if yours arrives slightly dented or with a scratch over the Apple logo, that's unsatisfactory, even if it works fine.
If it doesn't work fine, and a quick call to the dealer's or Apple's helpline doesn't reveal a simple solution, reject it. Never, ever accept repairs to a new machine; send it back.
Refurbished or second-hand goods, by their nature, can be expected to have some small cosmetic flaws (though refurbs usually won't), but beyond that the same rules apply. If you see the goods before buying, you're deemed to have accepted any visible defects, along with anything mentioned in the description, so take care and have a good look.
You can't normally reject non-faulty goods just because you change your mind, but the relatively new Distance Selling Regulations (DSR) do give you this right when you buy without visiting the seller's premises, for example over the internet. You have seven days from delivery to cancel your order. Who then pays for the return of the goods is a grey area; to avoid any damage-intransit disputes it's best to invite the supplier to collect, but they can take the actual cost of this – not a made-up number – off your refund.
When you reject goods or cancel under DSR, the supplier can't legally impose any other charges, such as a 'restocking fee'. You must take care of the goods while they're in your possession, but you don't have to return the packaging in saleable condition; it's not unreasonable to open the box.
There's no obligation to check the goods before signing the courier's paperwork, though it's wise not to wait very long, since your DSR rights are ticking away.
You can still reject the goods up to six months later if you find a fault. After this time, it would be up to you to prove it was an 'inherent defect'. A report to that effect from an independent Mac repairer should suffice.
If you're buying for your business or from an individual, you still have the right to get what you pay for and not be cheated out of your money, but the lack of statutory assumptions in your favour can make disputes trickier to resolve. Partly for this reason, and also because of the slightly dodgy rules that apply for historical reasons to auctions, eBay and its favoured payment system, PayPal, operate their own safeguards for buyers, which you can read about here.
There's no such safety net when you buy a Mac from your sister-in-law's mate Jeff's friend, Dodgy Bob, and give him the cash in a pub, so our advice would be not to do that, despite his competitive prices.