Not a bloody clue!
It's not something I do a lot of.
Seriously, in my case it's a matter of not losing money. I don't refurbish kids' bikes, for example, because the parts cost the same as full size bikes. A couple of tyres and tubes, new cables, grips and the like and suddenly you've spent more than the bike's worth. Given that kids' bikes - especially boys' - tend to have a hard life cosmetically, it's impossible to recover the costs.
In your case I would take the original cost and reduce that by 50% for the first year, then 30% per annum. So if the bike was 800 quid new and you've had it for 3 years. That's 400, 280, 200. (ish)
Then add in the costs of maintenance, refurbishment and any upgrades you've done. Make an allowance for what makes the bike unique and interesting. Do an ebay, Gumtree and *other* forum search for similar bikes that are being offered. Ask yourself what is it that makes *my* bike better (or not) than *that* bike. It may be cosmetics, it may be *yours* is unique, it may be upgraded over standard.
List the features and benefits, it may be super comfy over long distances, it may be outstanding as a commuter, etc.
Pitching a price is hard. Too high and everyone laughs. Too low and everybody wants to know what's wrong with it. Generally if you're within 80% of the highest you can find for sale that's a good ballpark to attract someone who wants a bike like yours. Most importantly don't pitch for what you want. Negotiation is a dance that everybody hates but has to be done. If punter feels he's getting a bargain you have more chance of selling so pitch it higher than you want and take a lower bid. No one EVER sells their bike when they put 'no offers' in the ad . . .
HTH