I have worked with over five startups. One in the cycling industry, one in the woman's fashion industry, an international coffee brokerage firm, a company that designs and produces its wallpaper, an engineering firm and others. You can find examples of my work at Seeking Alpha.
It depends if the price of your product is still competitive with other products or if your private label offers a benefit that other labels do not. I dealt with this with a commodity product here in Brazil. There is a formula I use to calculate the sales price after including margin and import duties. Please contact me if you would like more specific information.
I would not monetize this idea at first. I feel your platform needs to be able to provide this service prior to trying to charge companies to list their services. I feel this way for two reason, I as a customer would not trust the platform as I would believe that it only exists to pressure me to contract the services of the companies that paid to be there. I as a company would not want to put my company's reputation at risk prior to the platform having a good reputation on the market.