The problem is if you ship something you can do it by the cheapest method, but if you buy something and need someone else to ship to you then they are going to need to use a fully trackable service with proof of delivery.
If they send and the item doesnt arrive ( or the buyer "claims" it didnt arrive as is sometimes the case), then you would want a refund or another one, and without a proof of delivery your seller is just going to have to refund or replace at their cost.
The post office tracked service is only fully trackable in the UK and then it gets to be a massive aggravation to try and trace abroad, and takes weeks, something you the buyer probably wouldnt be prepared to wait for. For your seller to try and claim off the post office is a waste of time, the amount of forms and hoops they have you jumping though just isnt worth it and costs the seller much more in man hours trying to resolve it they stand to get back. There is also a time period before they can claim, which is long after you would have been screaming for payment back or another item.
So the cheap option, although great for you, is not so great for the person responsible for getting the item to you.
We always use UPS, and get a reasonable deal through them due to the volume we ship abroad, but on smaller items if often not cost effective.
One option open to you that might be feasible, would be to get your parts shipped to someone in the uk, and then you can have them ship it to you by whatever means you like, this is sometimes cheaper, especially if you want a few bits and they can package them all up and send them together. We have quite a few overseas buyers you do this.