The Aircraft For Sale (AFORS) website offers free Internet advertising for individual UK based aviation enthusiasts.
From AFORS:
AFORS, is a Microlights and Light aircraft for Sale website which provides Aviation classified advertising on the internet where you can get access to tens of thousands of potential buyers. Registration is currently free. Placement of up to 3 adverts per month are free to individual users. You can create, ammend or replace your adverts all online in real time, including uploading up to 6 photographs and videos for each advert. Users can create up to 3 individual adverts per month, more than that, then you do have to buy credits.