I run marketplace operations, let me tell you the most common calculation mistake.
Getting rid of commissions can mean getting rid of profit. Because the commission you pay on marketplaces isn't just infrastructure, it's the cost of traffic. On your own site, you'll buy that traffic via ads, and ad costs are higher than commissions in most categories.
Do the math like this: what's the marketplace commission percentage? What's the estimated ad cost to get a sale on your own site? If you don't know the second number, try it for a month with a small budget, don't guess.
The real value of your own site isn't escaping commissions: owning customer data, enabling repeat sales, and building a brand. If you want these three, open it. If you're opening it just for commissions, you'll likely be disappointed.