Send them an ebay invoice and tell them to either use the "pay now" button on the invoice or log into ebay and use the "pay now" button from there.

You absolutely need full seller protection on this one, as you're selling in a very risky area to a buyer with a very risky profile.

Use advanced search to see what else they've bid on or are bidding on.

I would be posting express courier international if I were to use Australia Post, but I'm willing to self insure on lost parcels.

Check whether bullion is even legal to post to the country your buyer is in.

(If it's Nigeria then it's 100% a scam).