I paid 38 quid posted on eBay for a minted and perfect working condition Headshok Fatty Ultra, older model but had disc mounts, somewhere around 80mm travel, air spring and 5 setting rebound adjuster rather than the lockout on top. One of better buys considering that they get good prices on US eBay (somewhere around $100-200 depending on model). If You won't use it than there is no point in keeping it 🙂 Headshocks are pretty popular and if You put a good price than it will shift quick, old P-bone off my GF f500 was sold in under 10 minutes for 10 quid plus 10 postage. There is still quite a lot of F series bikes and someone might want to buy Your Fatty for spares or because it will be in better condition then the one on his bike 🙂
Depending on the condition and model You should get some cash for it but as with eBay You might not get the right buyer straight away but starting from at least 50 would be reasonable.