I don't know how you've reported them thus far, but you need to use the 'Report' button (there's one on every listing). That's the only way to let the correct department know.
If the seller is a UK business seller, you should also report them to Trading Standards.
You don't say why you need their contact details, but sellers rarely need to be contacted unless they're selling items for collection. If something's gone wrong with a purchase, all you have to do is read the Money Back Guarantee and follow instructions. There are links to it on every listing, in various other places across the site, and on your order confirmation.
@andysue
