I'm a big fan of the French charity thrift shops Emmaüs. A bit like the St-Vincent-de-Paul or Salvation Army stores in the US, Emmaüs is a great place to find used items for a steal, and they also help reintegrate people back into the workforce. But like most people, I rarely have free time to go rummaging around their various shops in Paris, so I was thrilled to find their new online shop, Label Emmaüs, which selects the best clothing, accessories, furniture, decor, electronics, music, books, children's toys to sell online with delivery available throughout France.
They also have a special category in the shop called Créations, which are "upcycled" items that are reworked, reimagined, reconditioned or customized by the Emmaüs team to give something destined for the landfill a new lease on life. If you're sick of having the same exact Ikea decor as all of your friends (in almost every Western country!), then this is the antidote.
I also recommend checking out their Geek-a-Brac if you are nostalgic for vintage electronics, or their regular electronics section for almost-new electronics smartphones, laptops, video games, and all sorts of accessories you need that are already "obsolete" and hard to find in shops. While writing this article, I casually scrolled through this section and saw the exact same Kensington laptop docking station that has been in my FNAC shopping cart for over a month (I've been waiting for it to go on sale; no dice). Not only is it listed as "new" on Emmaüs, it's 50% off the price (so I just saved €70), and there's a 6-month guarantee that it will work (and 15-day money-back guarantee for any reason I'm not satisfied with the condition).
If you still prefer shopping in person, you'll find a list of all of the Emmaüs shops in France here.