There are two huge mistakes people make when launching an eCommerce marketplace.

1. They don't have enough traffic to support the initiative.  They leave it off to the side of the site, languishing with buyers.  While they often do this for good reason - quality is the usual one - this kicks the can on the initiative, dooming it to failure.  If you aren't going to go out and get traffic for your marketplace, you probably don't need one.  Just dropship!

2. They have absolutely no idea how to attract sellers.  In a traditional retail company, third-party sellers are not a native concept.  Your seller organization (first you need one!) needs to start with a clear "why".  And not the why of your website.  But why for the sellers?  Many retailers are shocked that when they reach out to a potential seller (who is on Amazon) about their new site, they aren't interested.

Let me fill you in on something -- the seller is more savvy than you are - and certainly more savvy than the intern you are using to trying to pitch them with the horrific message of  "it's another channel, why not?".  They want to sell just as much as you do. 

It's just you haven't given them good enough reason to spend time with you.