Capitalist Trophy in Iron Sea Defenders

This is likely to be a trophy you need to consciously work towards, as selling units isn't something you will be doing frequently.

There are 2 options to approach this - one is to wait until the end and farm it, the other is to work on it as you go. I would suggest the latter, but the approach will be the same regardless.

The best method is to start a level normally, get your towers set up, and then once you have everything upgraded, let the last couple of waves run on auto. Use this time to select a build location that is a little out of the way, and repeatedly build a basic cannon, sell it (don't upgrade it), then build again. These only cost 35 gold if you have maxed out the cost upgrade, so you can easily farm 30 or more sales in a given level.

If you work on this as you go, whenever you have a level fully set up, then you can chip away towards the 500. Alternatively, you can do this while farming the 50,000 kills.