If you are trying to send a phone number, personal email address or your private address without the buyer buying or committing to buy an item then eBay will block your communication, since they see it as you or the buyer trying to sell off-eBay.
If you continue to try doing it both you and the buyer will face a one week ban on messaging anyone.
Just contact the buyer through the eBay messaging service.
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"Start me up I'll never stop......"
