My server and related capacities etc are way bigger than I need.
I'm wondering if rather than downgrading it might make sense to hire out some of the capacity.
Are there places people go to buy and sell server space?
Or other ways to go about this?
My server is in the US because that's where my biggest market is; though I'm based in Ireland.
It'd be easy to rent out space on it - especially if you have a control panel like WHM installed...but first ask yourself if you want all the potential hassles of essentially being a webhosting company. (dealing with abuse complaints, collecting payments from your clients, etc.) It might be better to just downgrade to a VPS, there are many providers that offer inexpensive VPS packages, and many will migrate your data for free.
Pingpipe more or less hit the nail on the head. It might just be more hassle than it is worth for you to rent space out