What are the main benefits of using international domains as opposed to .com and how do people use them?
i.e If I had a website that was pretty good in the UK and I wanted to sell abroad, could I just buy an international domain and then post my website on there in a different language with the orders coming through in the same way as my UK website?
There is no such thing as an "international" domain, any country as long as there is no block, can get to any other domain. It's for separating products and page design and languages. You can use one domain name for everything.
So yes, if you want to expand your service, buy a domain name with whatever country extension you want, set it up the same way you did your original one with localized content.