I have a bike that I have built up over the last few years. It's a good bike and works as it is, with a good quality components. However, I want to sell it. Is it best to sell it broken down as parts or as a full bike?
It may depend on the overall age of the bike but I would guess that in most cases it's better to sell it as a whole. If the bike only consists of rather standard components that one could easily buy at a LBS, then they wouldn't make high prices and the additional work to break it all down and maybe set them all on on ebay separately may not be worth the money.
I can imagine only two cases where it may be better to sell it in parts or at least to sell some parts separately:
Based on my experiences and assuming that you plan to use eBay, I would most definitely sell it in bits for this reason: No one will be searching for your custom bike config, so you'll only ever receive interest on a general level, and never benefit from targeted bargain/collectors frenzy :)
eBay is busy enough now that most items you can think of listing will have a specific audience. There WILL be someone searching for a [insert you particular chainset] or [insert your part-used front tyre] and that person will develop a vested interest in your item because they "need" it to complete their own custom bike, or to replace an exact part on their beloved shop-bought complete bike.
I've done this a lot, both as a buyer and a seller. eBayers will always bid over the top for something they "just want".