Mid Century Modern Market at Drill Hall Edinburgh
Gumtree, EBay and Newsnow
EBay tends to be a bit more expensive, but has a bigger range if you are prepared to travel or have the item delivered without seeing it in person
A great site I love is Newsnow, as it trawls both Gumtree and EBay, along with other classified sites. I was getting really irked by the ads that I'd accidentally click on when scrolling listings on Gumtree whilst on my iphone, so this site thankfully alleviated that.