There is no number. Anyone giving you one — including a marketplace — is guessing or selling something.
What there is: the sum you can do with your own stock, which is more useful than a figure from somebody else's business.
The arithmetic
Start from what a thing realistically sells for, not what you hope. Then take off, in order:
The commission, worked out on the item rather than the delivery. The live rate is on your own Seller Tools screen, and it is set per store — which is exactly why no blog post can state it. When is the commission taken?
What the item cost you.
Packaging. Real money and routinely forgotten.
Delivery you did not fully recover. Check what the courier actually billed against what you charged. How do I work out what to charge for postage?
Refunds and returns, averaged over a few months rather than per order.
What is left is per item. Multiply by what you can realistically sell and pack in a month.
The number people forget
Your time. Photographing, describing, hosting, answering, packing, posting.
If the hourly figure that falls out is below what you would earn doing something else, that is worth knowing early rather than discovering in month six. It may still be worth it — for the flexibility, for building something of your own — but it should be a choice rather than a surprise.
What actually moves the total
In order of effect:
Sell-through, not volume. Listing more things you cannot sell adds work and no money. What is sell-through?
Buying better. The price you buy at decides more than the price you sell at, because you control it. Where do sellers find stock?
Not losing money on delivery. The commonest quiet leak, especially on bulky items. Why does a big light box cost so much to send?
Repeat buyers and followers. The only thing that compounds.
Fewer returns, which come almost entirely from descriptions that flattered. How to write a condition description
What does not move it as much as people think
A boost. It multiplies whatever your show already is. A show with nothing happening gets more people leaving. Do sellers pay to appear higher?
More listings, if the existing ones are not selling.
A realistic shape
Quiet for a month or two, then slowly compounding if the fundamentals hold. Anyone describing something faster is describing an outlier. What does a realistic first month look like?
More: what it costs to sell, what comes out of a sale, is selling live worth it.