I am trusting that other people will add or correct this:
All should be explained in the various stickies found at the beginning of this forum. In brief you need to have made a minimum number of posts before access to sales is granted.
Other sources: Shooting The Breeze has a sales section but I do not know the conditions. If you go to Google and enter a model you are interested in followed by 'for sale' you may well get some hits. Local Club? Might be worth asking. Auctions? a possibility although there is a risk the item being auctioned is in need of attention.
But, whatever, welcome.
Cheers, Phil