I described here is not enough for all those who want to see and touch stuff before buying. There are though several 'real' places where you can buy used items - clothes, kitchenware, small furniture, spare parts for everything you have at home, old books, framed pictures, bicycles, skis, electronics and lots of other second-hand stuff.
Finnish flea markets (kirpputorit) are quite specific places - frankly speaking, they don't look very attractive as you can see at the pictures (taken at "Centrum-Kirppis" in Jyväskylä). But many people, especially students, often visit them looking for original old-fashioned clothes or just cheap way of equipping their dormitories. So, if €8 per a simple mug is more than you'd like to spend - head for the nearest kirpputori and you'll certainly find everything you need!
A dedicated kirpputori-search engine is http://www.kirpputorihaku.com - here you can find local flea markets for any town in Finland. It says there are 24 second-hand shops in Jyväskylä!