I know that the thought of wearing used shoes makes some people nauseous, but I've never taken issue with it. In fact, probably 1/3 of the shoes I own are secondhand. In a consumer culture dominated by 5-week trend cycles, people are doomed to impulse buy, which leaves a lot of gently used stuff on the secondhand market to trawl through.
I've typically thrifted or searched ebay to find secondhand shoes, but the popularity of online shopping has resulted in all sorts of new places to shop for secondhand goods.
I discovered Tradesy
over the weekend. It's a mashup between the Ebay/Etsy, direct-from-the-seller approach and more curated sites like thredup and Twice. Basically, an individual lists items, which are placed both within their personal shop and within the larger marketplace. Once an item sells, the Tradesy team processes the order and sends the seller packaging materials, which the seller then uses to ship the item to the customer. Items ship free and returns are free, as well.
Tradesy's setup isn't perfect. Items that sell out are delayed from removal while the site waits to confirm the order with the seller, which means the customer has to scroll through a lot of extraneous listings to find something she can actually buy. But they're the best I've found when it comes to shoes. Prices tend to fall in ebay ranges with the advantage of avoiding the auction and having your item ship free.