It depends on what you are selling, and if you do it by yourself, or you will send the items to someone to sell for you. 3 sites that sell clothes for you, doing (most of) the job, and paying best for your old clothes are:
1) Tradesy (flat commission fee of $7.50 for items under $50, or 19.8% for items over $50). 2) Poshmark (for sales under $15, the fee is $2.95, if $15 or more, the fee is 20%). 3) Flyp (you will get 50-70% of the profit)