Speaking from my own personal experience as someone who has been in the art world my entire life: there are two types of buyers for "decorative art". 90% are looking for something very specific such as an image the Golden Gate Bridge, puppy dogs, purple irises, local landmarks, etc. The other 10% are impulse buyers that love an image and buy it. I'm new to this site and quite frankly don't expect much. FAA is just another venue for me. I'm represented by another "traditional" gallery that sells online; they require artist's offer signed numbered edition prints / originals that you produced yourself, before you can offer prints... I get sales without advertising because the site draws collectors that want a limited prints or an original. For Fine Art America I'll be working on refining my descriptions and keywords, as well as the images I post. It's a combination of having specific images people are searching for, a good description and well thought out keywords... and you can get sales without much advertising.
As a point of art snobbery, I don't use social media at all; I feel it diminishes an artist. Besides that you can get plenty of sales without it. Direct sales work, print up some postcards with your info and drop them in every mail box you see.