The two popular ones are Poshmark and TheRealReal. Both make it dead simple to sell (and buy!) luxury items. The RealReal will pay you 60% of the sale price (more if you sell a lot) and Poshmark has a tiered commission structure. Sale prices under $15 pay a $2.95 flat fee and everything else is just 20%. Tradesy is another company and they take just 9%.
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.