Daft question

[Saltire]

If I list osmething, can I stipulate only posting to UK? and if someone bids from overseas, can I not sell it to them?

have had 3 overseas buyers claiming that their parcel hasn't arrived

Yes it's all in your settings thingy. Means that people outside of where you allow will not be able to place a bid.

Make sure you tick the box in your buyer requirements. If you dont, they can bid/win even if you state UK only, and then you have to eithe rpost it or have a non-performing seller dispute on your hands