When you first learn TFT, one of the easiest rules to understand is:
“If I'm at 9g, sell this unit so I can reach 10g.”
10g gives 1g of interest, 20g gives 2g, 30g gives 3g, and so on. The econ breakpoints are clear.
But if you think only about hitting those breakpoints, you can sell units in Stage 2 that you will want for your Stage 3 board.
Before maximizing interest, learn to hold potentially playable units on your bench.
Do not look only at the board you are currently playing. Think about what you want your board to look like in Stage 3.
Level 6 is especially important. You often reach it around 3-1 or 3-2, and you spend a large part of Stage 3 there.
Going from Level 5 to 6 also increases your board from five units to six. Proportionally, that extra slot changes your board more than going from six units to seven.
So while looking through Stage 2 shops, ask:
When I hit Level 6, which six units could I play?
Hold units that might be useful on that board.
This is the idea behind what the Japanese version calls “warming the bench.”
When you gain an extra board slot, think about which unit would actually make you stronger.
Do not look only at trait counts.
Does your board need more damage? Or is your frontline too weak, so your carry does not get enough time to deal damage?
Use those questions to identify units that may be useful at Level 6.
Do not immediately sell a unit for interest if there is a realistic chance you will use it at your next level.