What is the best way to sell complete used LEGO sets?
Are you sure that your sets are complete? You should verify that no pieces are lost, either by building the set or by checking LEGO's parts database (can do that on Brickset.com). If pieces are missing, you may want to order these.

On BrickPicker.com you find current market values of sets, so you know what asking price to set. That site collects eBay price data regularly. Then select marketplaces; eBay is used a lot by LEGO investors and it has an international audience of buyers. When it's time to ship your items you need to take care packaging the sets properly. To some buyers the condition of the box will be of great importance.

As you can see there's a lot of work involved with LEGO selling, and if you want to make serious profits you need to get economies of scale in your business, not just selling one or two sets.