Unless you have their email address you need to send any messages through eBay's system. If you try to contact them through eBay messaging trying to get their personal email address eBay is likely to see it as an attempt to organise an "off eBay transaction" and both you and the other party will be sanctioned by eBay, and prevented from sending any messages to anyone for 7 days in the first instance.
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"Start me up I'll never stop......"
