Sam,
Certainly you can advertise the bike here. I am sure there are a number of Douglas enthusiasts that would love an 80Plus model. There is a board specifically for listing Douglases and Douglas parts for sale here:
https://www.douglasmotorcycles.net/index.php?board=3.0Since you are already registered for the forum, you can create new posts in the 'for sale' board. Take care of any personal details, as anyone in the world-wide public can read the contents of the post. So do not include your personal email address, as spammers and autobots can mine the public posts for these. Have interested parties contact you via the forum Private Message system (they have to register), a reply to your post (same applies), or some folk prefer to use a phone number.
Also you can place an advert in the London Douglas Motor Cycle Club's magazine. In this situation, you do not need to be a member to place an advert, as they are always happy to see another Dougie connected with an enthusiast and potentially future club member! If you want to pursue that, I can PM you the contact details.
-Doug