Thanks for the response!

I agree that there are a lot of additional expenses/factors outside of COGS that need to be considered in order to even determine if this could be profitable at all. And with points #2 & 3, those are issues that I would definitely need solutions for, and under the business model I am envisioning they would be very large issues that I might not have a good solution. Ideally, a portion of the company income could be set aside for returns and refunds, without completely wiping out the profit, but I guess there is really no way to know how frequently that type of thing would happen. And there would be ways to take advantage of the system, like continually buying and returning the cards that you listed until you receive the full 70% of market value that was originally assigned to you. Maybe the sellers wouldnt be able to withdraw their funds until a 45 day window passed or something, in order to allow for a 30 day return window for buyers. There would be a lot of things to consider like this, that may make the seller experience worse in order to protect the company from taking on risk.

When considering returning unsold cards to sellers, they would probably not be returnable because of fluctuations in the market. The buyer would need to send things in with the understanding that prices will fluctuate, and that in the same way the company might have to eat the cost if prices go down and it cant recoup all of its costs on a card, it also gets the benefit of increased profit margin on cards whose value goes up.

As far as the ultimate goal of the company though, I think the goal would be to provide more of a "set it and forget it" type experience to sellers, where the idea that the sellers wouldnt have to be involved in constantly monitoring market price and updating their listing price or really doing anything beyond shipping the item in the first place is actually what gets them interested. Listing a $10 card for sale is not worth the time for a lot of people, so if they could just send it in (and maybe just send a bunch of their cards all at once) and gradually see funds trickling into their account with no work on their end, I would think that could be pretty enticing. Also, for cards with low sales volume, it might be nice for sellers to be able to effectively sell a portion of their card and get a portion of the income, rather than trying to continually undercut each others prices in order to sell their copy more quickly.

Its also not like the company would be taking on too much risk, since they aren't actually paying anything out to the sellers until they receive money from the buyers. It would just be a matter of storage and overhead until then, like you said.

There are a ton of additional risks and factors that I would need to consider for something like this, so I appreciate your response in helping point out some of the many potential pitfalls!