Pico-8 numbers are "16.16" fixed point values, rounded to four decimal spaces. 16 bits for integers and 16 bits for fractional values (I don't know how the fractionals work exactly, but the game only has integers for cash values anyway). https://pico-8.fandom.com/wiki/Lua has more details.
So that's 2^16 = 65536 possible values, but for the integers that includes 0 and negative values, so:
>print(32767+1)
-32768
For a range of numbers from -32768 to 32767.
(I noticed just now that the range is uneven, with more negative numbers. I guess they have to include the zero so it has to be uneven.)
So I got to 32767 by trading and then +1 over to -32768, which doesn't fit the screen. Of course I couldn't buy anything anymore, but selling would count towards 0. I don't think it's possible to get back there though since the planets/systems probably don't have enough money at that point :)