I'm looking to start a business similar to One Kings Lane and not sure where to start in terms of sourcing products from manufacturers. Is there a site where I can see a curated list of merchants and their products and then somehow negotiate a discounted wholesale rate?
The first thing they do is scavenge for the best brands with great products. They have experts on the niche that do this for them. You can hack that approach by using Google Keywords Tool or Reddit for researching which brands / products to work with.
The next step is to contact these brands to:
Due to the distribution that One Kings Lane and Gilt have, they require the brand to block out the inventory before the sale starts on their sites. Unless you have large scale distribution like them right off the bat, it'll will be tough to do that. Which is fine, you can worry about that later once your sales reach that point.
One Kings Lane then starts the sale on their site. It's only after the sale ends that they place the final order with the vendor (i.e. they first get payments from customers before buying their inventory). The terms are usually net 15.
The real profit in the flash sales model is in moving inventory that the brand wants to get rid of (remnants). The vendors discount their product 60%+ as compared to wholesale and One Kings Lane uses that to create high margin value on their end.
Both One Kings Land and Gilt handle logistics as well (which is costly for someone new). Your best bet is to either to use Amazon Fulfillment (I've used them before and they're wonderful, albeit costly) or only pick vendors that dropship. You want to spend your time growing sales, not packing boxes.
Shopify has a great guide on dropshipping you should read.