Your first option is the second-hand resale stores like Buffalo Exchange and Plato's Closet. At these stores, you bring in your unwanted items and a store associate evaluates them and comes up with a resale value while you shop around. Find out what's trending before you go so you can get the most bang for your buck. You get more of a value if you accept a store credit to buy new clothes, or youcan take your cash. I'd recommend the store credit because there's always some gems that you can't buy brand new in secondhand shops. One man's trash is indeed another man's treasure.
A rather new and great way to sell your items is through poshmark.com. Poshmark operates mainly through an iPhone app. It is an online community of buyers and sellers where you can list your items and people can like, share and buy them. You may be thinking what makes it any different from ebay. Well with Poshmark, you get to connect more with other users and the company covers all fees, shipping and postage at no cost to you- the seller. You can also negotiate prices and create personalized deals and trades for other people's items that you may like. Once you make a sale, you receive your postage via e-mail, and all you have to do is print it, attach it to the package, and send it off. It's a great tool, if you decide to register, let them know I sent you by signing up with the code HODHR. :)
Just a little added info that's kind of off topic in true Kaitlyn fashion, in case you're looking for something but have absolutely no idea what it's called, who made it or where to buy it - there's a solution for that as well. Wheretoget.it is a site where all you do is upload a photo and the web community provides you with links and prices to where you can find and purchase it. I'm on that site as well, so if you sign up, follow me.