There are plenty of second hand stores in this area. The trick is knowing where they are. This is a listing of some of your options. If you know of others that you would like to share, let me know in the comments or through email (hereatgk@gmail.com).
Werk aan de Winkel Sittard (Mostly clothes)
Putstraat 30, Sittard (Close to Bagels and Beans)
Tues-Fri 1000-1700
Saturday 1100-160
Kunterbunt Second Hand Kinder Boutique (All things related to children)
An der Burg 8, Gillrath
Mon 0830 – 1230 & 1400 -1800
Tues, Wed, Fri 0830 – 1230
Sat 0930 – 1300
Closed Thursdays
Lupalums (Children clothes, books, toys)
Sittarder Str. 8, Gangelt
Mon-Fri 0930-1130
Mon, Wed, Thurs 1430-1800
De Goeden Doelen Winkel (Furniture)
Nusterweg 88-C, 6136KV Sittard
Tues – Sat 1000-1700
Meubel Depot (Furniture)
Dr. Nolenslaan 151, Sittard
Mon – Sat 1000-1700
Bis-Bis (Furniture, clothes and everything else)
Milieuparkweg 13, Sittard
Mon 1300 – 1700
Tues-Fri 1000 – 1700
Sat 1000 – 1600
RD4 (Furniture, clothes and everything else – two stores)
In de Cramer 9, Heerlen and Spekhofstraat 4, Kerkrade
Mon – Sat 0900-1700