Well, I`m not a numismatic insider, but I heard of those. AFAIK its not a 999 gold coin, only 900 or so. And I dont know what size it has either.
You have to assess what you get for the money. If its a collector's item price, the condition will be very important and make a big difference in price.
In my opinion (and thats purely non-professional advice) better stay away from that collector stuff unless you really know all the ins and outs of those coins.
And even if you do know the matter, its still always difficult to find the right buyer for your collectibles when you need one. Its not like selling the pure metal, for which you have price averages everywhere.