We run a one-sided fashion marketplace where we need to increase the number of deals. We know that sellers also buy things (seller are buyers). They are really motivated to sell their items. What if we create a "special category" on our site where you get in if you buy something from other sellers in this "special category", and than you get immediately out if you sell something (until you buy something new again). In this case we could create a perfect consumption loop. We want to study this practice more before we run into a qualitative research and MVP.
(I'm the founder or Clarity, and have spent hundreds or hours talking with founders from 20+ marketplaces like Thumbtack, AirBNB, Fiverr, etc)
It sounds like your challenge is getting more supply (sellers) but I'm assuming you want to keep the quality high - hence the idea of seeing if they're buyers first. I get where you're going with this, but here's my thoughts.
You're customers come to you because you made them a promise on your home page. Maybe that's to find the coolest underground fashion, or items from independent designers - regardless, that's your promise and the key is to deliver on that. Most marketplaces (almost all) curate their supply to ensure 3 things 1) quality, 2) uniqueness (aligned with the promise) 3) liquidity (supply must promote their listings for you to scale). So think about that problem and the best way to solve it - I'm guessing asking sellers to first buy may not be the best way to acheive 1,2&3.
THE GOOD NEWS
Most marketplaces I've seen, Clarity included, sells to their supply. It's usually 20-40% of the transactions come from someone who's also on the supply side. So, naturally, you could start sourcing supply from your buyers IF they fit the promise you've made on your home page.
THE BETTER APPROACH
Focus on providing a way for your sellers (supply) to make a meaningful living ($1500-$2000/month) on you site. If you do this, you'll attract other motivated sellers that will also help you provide liquidity by being responsive AND promoting their listings to their network.
If a call would help you think through it more, I'm available within a day or two. Feel free to post a request.