I just received this email from an eBay seller.
As there are only 3 known of these I am tempted to regard this as BS :ok:
hi there
i have many gold coins for sale including 1887 five pound gold sovereign for sale at $3000 sold one recently for $3300 check my feedback if interested i can send you a list with prices
I got that email too.
Spam.
The e-mail might be spam, but there are "plenty" of 1887 gold £5 coins around. The scarce ones (with only 3 known) are from Sydney mint. London mint made 54,000 for circulation and 797 proofs. Here's the NGC listing (http://www.ngccoin.com/poplookup/world-coin-prices.aspx?category=123043&worldcoinid=315417&FromSearch=true).