Just wondering why just the CPU and not a core bundle-most of us would upgrade the lot in one go and sell the CPU/MB/RAM as a bundle to help with the upgrade cost.
Yep, about £30 seems reasonable, but use Gumtree rather than E-bay, they'll advertise for free-although you can add features for a fee-and you can usually deal direct with the purchaser, a simple cash deal that incurs no fees.
EDIT: Aim for <>£60 for the bundle but you may need to accept a little less, and don't forget to ' sell ' the parts, it's an old setup but plenty capable for a for home/office user, add a Blu-ray player and it's the perfect core for a dirt cheap HTPC for example.