Statistics
Not «how much did I take in». «What should I buy next».
An Excel sheet can work out revenue and profit too. Vintaflip works out the numbers that change your decisions: real margin by channel, turnover days, cash and break-even.
Basic statistics on Free · advanced from Premium

P&L by channel
The real net margin, channel by channel, fees and shipping included.
What you get out of it
eBay gives you volume but on Vinted the margin is 59%: maybe your next clothing item shouldn't go on eBay.

Turnover speed
How many days a category sits in stock before it sells.
What you get out of it
Trading cards turn in 6 days, mid-century pieces in 74: the same money invested performs very differently.

Real cash
Taken in minus spent, month by month, with tied-up capital worked out separately.
What you get out of it
You have €852 sitting in stock: you see 300 in the bank. It isn't a loss, it's capital you haven't freed yet.

Break-even and long tail
Which lots have broken even and which items have been sitting too long.
What you get out of it
Eleven items sitting for more than 60 days are €240 of capital: maybe it's time for a price cut.

Basic or advanced
What you see with which plan.
Basic statistics · all plans
Advanced statistics · from Premium
You already have the numbers. Reading them is what's missing.
Log your first ten sales and the statistics start talking.