I very rarely recommend display ads for user acquisition (and I'd be hesitant to call this a growth hack!), but because your product is very visual in nature and you're targeting users of one specific site, it might be worth testing an ad campaign through the Google Display Network, using managed placements to only display the ads on ebay.com. (More on managed placements here: https://support.google.com/adwords/answer/2470108?hl=en)

I'm guessing there are also plenty of forums and sites that ebay sellers use to discuss best practices and such - I'd do some research to see where your audience hangs out and start participating in the conversations there, ideally in a way that is more focused on being helpful than being salesy.

Let me know if you'd like to discuss further & I'd be happy to talk through some more ideas!

If you are able to track or follow the sellers contact info or twitter handle. Send them a link with the pre populated link of how their offering could look using your service. Owler.com has an interesting way of tweeting out someones competition that way if the owner of a specific startup is tracking their brand they are drawn to their pre built link....your product could resemble this tactic. In essence you plant a seed and if the rightful owner sees it, then they may claim it. Show it built first, then sell it ;)

What's your budget?