[octbash]

The quick answer to your broader question is that there are multiple ways of slicing how GDP is computed, and yes, economists are aware of them and have thought through the edge-cases as well as being aware of where the measures fall short.

E.g. in your example, see the Value-added approach: https://quickonomics.com/gross-domestic-product-gdp/

[galaxyLogic]

A good resource thanks. "It measures the total value of all goods and services produced".

But so then if I produce nothing but just sell something at profit which I already have is that part of the GDP?

If the value of my stamps-collection increases and I sell it at considerable profit is that an increase in GDP?

[PeterisP]

In general, the added value would be part of the GDP - e.g. a retailer would be considered to contribute its revenue minus the wholesale cost of goods, rent, salaries, etc which was already counted as produced by its suppliers.