Selling damaged units and replacing them

Well, remember that you won't sell certain units. Once I research vertigos, I generally keep 1 high level and never sell it.

So early game, I'll have around level 14-15 CC. I'll have 3-4 units I'll never sell:
* Level 15 vertigo
* Level 15 venom
* Level 15 scorpion
* Level 15 militant or reckoner (kinda depends on how the bases are)

A level 15 CC gives 110 army points. Out of the 110 army points, I have 35-40 points tied up with units I don't sell. The remaining army points are flexible and I'll add level 7-10 army units depending on what the base defense calls for.

That should satisfy your requirement for not always selling everything and losing out.

It is standard practice on FA worlds and really any non-morale world. It's used aggressively early on, and slowly goes away after switching to a new main and when the Command Center has been leveled to gain all army points.

Generally, you will keep 4-5 army units very high level. I call these takedown units. The rest remains flexible which helps to tailor each run on a Forgotten base to maximize damage.

I generally stop buying and selling the majority of my units when I research cobra + shields and when I level my Command Center to get all army points. Everyone is different though.

And yes, it costs crystal and power. Each time you buy and sell it's a 10% reduction. It is very helpful to have super high power production. Shooting this way creates more base kills and I have never lacked crystal. Most people trying this technique out usually run out of power which frustrates things. So, always have a lot of power production on hand.

In terms of levels, you can get away with level 7-8 units until level 23-24. After that, I switch to a hybrid approach. I'll sometimes level units to level 9-10 or a little higher. Especially if it's an attack that I'll use multiple times in a row. Then I'll sell the units necessary to make a better attack. I'll start using the RT I have saved up a bit over the first few days. Generally, the RT buffer lasts me until I switch to my main base which I usually have saved 5-7 days of RT in.

This is my way of buying and selling. It may not be the best way. I hope more players chime in.

Well, this is certainly interesting. To me, I only sell the unit if it requires 300 to 400 crystal worth of repair time. Any cheaper than that, I would ignore. I also worry about selling vertigoes if they are at a high level and I have few crystal... do it wrong and the vertigo returns with a lower level. Sometimes when I have 2 vertigoes of either the same or similar levels, I'd sell one just to upgrade the other.
I'm just simply looking for opinion as selling units, replacing them and upgrading them feels really risky as you can sometimes have insufficient crystals after an attack.