If you've ever sold a prized but overworn dress at a consignment shop or found a designer gem at the local Salvation Army, you know that secondhand shopping is for the most part an anonymous experience. As a buyer, you know nothing about the previous life your new find lived; as a seller, you're just hoping it finds a good home.

The world of clothing swap and resale websites is a different story, and those sites are gathering a wealth of data on the life spans of different designers. With communities in the thousands, these startups are able to say with conviction that if a woman on their network is buying Alexander Wang, she's also buying Acne and Theory. Similarly, they can track how soon after its original retail launch a certain item shows up on the secondary market - that is, how soon after purchase customers are unloading different brands.

In short: consignment sites might help us build a more complete picture of what happens to a piece of clothing after the initial buy. So I got in touch with a few founders to see what trends they've been seeing.

Tracy DiNunzio, the founder of Tradesy, said her team has found that for women who purchased an Hermès bag on the site in the last six months, the other brands they most frequently purchased within that same time frame were Zara, Gucci, Hermès, J.Crew and Louis Vuitton. In fact, DiNunzio said that Zara crops up frequently in relation to luxury brands, including Helmut Lang, Prada and Fendi. The takeaway: high-low shopping is alive and well on the resale market.

Another tip? If H&M's next monster-hit designer collab sells out at your local store, try looking on a resale site. According to Poshmark founder Manish Chandra, the retailer's Isabel Marant collection appeared on the site within hours of its release in H&M stores. Some of those sellers are doing so to make a buck (smart), while others don't want to bother with returns or to monetize a single wear.

"It allows you to have a very different relationship with your closet, as opposed to buying and keeping it there," Chandra said.

Sari Azout, one of the co-founders of Bib + Tuck, said that they've also seen pieces from the Lanvin for H&M collection sell for more on her site than they originally retailed for.

And if Rag & Bone is listening in, Azout has a suggestion: consider a store in Ohio.

"If Rag & Bone were to open a new store, there's a big market in Ohio. We sell a lot there. I don't know why," she said.

Louis Vuitton, Chanel, Lululemon, Rebecca Minkoff, Tory Burch, Kate Spade and Free People showed are the bestselling brands on Tradesy, indicating high demand on the resale market. On the flip side, brands that show more supply than demand include BCBG, Elie Tahari, Banana Republic, Ralph Lauren, Calvin Klein and Forever 21.

That said, BCBG is still a top 20 brand for Tradesy, meaning there's strong demand but still significantly more women looking to offload it.

DiNunzio pointed to Rebecca Minkoff as a brand that's an anomaly on the secondary market. The average time between the initial purchase and date a Rebecca Minkoff handbag is listed on the site is between four and six months, meaning women are buying them and wearing them for only a season or so.

But Rebecca Minkoff also has a 72 percent sell-through rate - meaning there's demand - and they only sell for 32 percent off retail price. The takeaway: maybe the Minkoff shopper has a few investment bags from other brands for the long-term, but buys the designer's bag as a seasonal wardrobe update.

While this data is fascinating for any fashion fan, it's of even more interest to brands. If you're a retailer or designer, information on customers' shopping behavior could help make decisions on everything from design to targeted advertising on Facebook to pricing.

Of the resale sites I spoke with, nobody had cut a deal with a brand or retailer on packaging data yet, although there's been inbound interest. DiNunzio said that her team has been talking to a few brands and one major online fashion brand, and they're close to closing a deal.

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