All the usual eBay tips apply, such as checking the seller's feedback, and you need to read the description very carefully: many used Macs are sold in non-working or damaged condition, or with different features or Mac OS versions than you might expect. Take your time and you'll find the right deal for you.
Besides deciding who to buy from, there's the thorny question of which Mac to pick. When Steve Jobs returned to Apple in the late 1990s, he ridiculed the bewildering array of products and quickly whittled it down. Since then, it's crept back up. You can have a MacBook, MacBook Pro, MacBook Air, Mac mini, iMac or Mac Pro.
Within each range are several standard models, plus built-to-order options that add up to hundreds of possibilities. By comparing the prices of different configurations – easily done at http://store.apple.com/uk – you can suss out the best value.
For example, at the time of writing the cheapest Mac mini costs £510 (all prices include VAT). It's a great little machine, but its 160GB hard disk is small by today's standards. The next model up has 320GB, and also comes with a slightly faster processor and 4GB instead of 2GB of RAM, for £153 more.
That doesn't sound too bad – but consider the alternatives. You could start with the base model and plug in an external 500GB hard disk for around £70, giving you twice the storage and enabling you to use Mac OS X's Time Machine for backup.
If you need the extra RAM as well, you could buy 2GB from a reputable supplier such as Crucial for around £45 and fit it yourself. So, except for missing a small performance boost, you could have a more flexible system for £40 less.
With iMacs, things get more complicated. The entry-level 21.5-inch model is a bargain at £969. If you want a higher specification, the next option doubles the hard disk space and adds a more powerful graphics card – but at £256 extra, you'd really have to want that graphics card. Once you consider this configuration, however, your eye may stray further to the third model, which offers identical features with a massive 27-inch screen (that's 50% bigger in area) for just £153 more. Considering you'd pay upwards of £500 for a 27-inch monitor of this quality, that's almost irresistible.
Then again, if you're into games the 256MB ATI Radeon HD 4670 graphics may still not quite satisfy you. The top-end iMac ups this to a 512MB HD 4850, but again the price hike is a steep £256, partly because the processor is also upgraded to quad-core. If you don't need quad-core – and only a limited range of tasks will benefit, after all – that's frustrating.
But hang on: you're not limited to the preset models. By clicking the Select button, you can tweak the specs to create your own Mac. In this case you could pick the base 27-inch model and upgrade just the graphics card for £123. Not every item can be swapped, but it's well worth checking the options.
Build to order is also available from Authorised Resellers, who may add third-party components for you. The only downside is that you'll need to allow a week or two for your own personal Mac to arrive. Just think of it as an exercise in delayed gratification.