Noel, your issue is different as you are registered and residing in Ireland, you can list on any ebay site worldwide without issue.
Slipster, if your account is registered in the UK and your listings state that your item(s) are in the UK, and your ip is showing that you are in Spain, then yes that could become an issue, my advice would be to teach your sister to list if she doesn't know already.
Remember when dealing with an unscrupolous company like ebay, I always advise to avoid any unneccessary attention by flying under the radar.
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