What is the general rule for deciding how much a 'used' bike is worth?

See what others sell for? Best guess? Gut feel?

When spending cash on other 'similar' priced items (motorbike, car etc) there is a wealth 'For Sale' to benchmark against. Any dealers out there, how do you price a trade in for example?

So what is the rule of thumb for a bike?

Then I suppose there's the question of sell as a whole or as separate parts………….

Ebay's completed items is your friend, or even try searching the classified here and on bike-radar for ideas on pricing.
The "sell it whole or as seperates" purely depends on if you want to maximise your return on the bike but take into account the time take to advertise (cost of advertising) and post and pack the items.

at the end of the day Oranges 5's sell well, if priced right.

Lots of decent photos will save a essay of description.

Oh and the rule of thumb is supposed to be anything 2nd hand is only worth 50% of the retail price, -10% for every year, but with a sort after item (say an Orange 5) you can happily ignore this rule.

I sold an 2007 Orange 5 on here last year for £1250 as i recall. Specced with Fox 32 Van R 15QR, SLX gears/shifters/cranks and DT E540/Hope wheels.