Is this considered cheating?

Yak 40

No, it’s free market capitalism at its best.

I prefer an airline investing in another one because this is not realistic.

it is borderline, technically not cheating unless the accounts are operated by the same person.

Moving forward (after the next reset) - aircraft auctions will be system-generated and user sales will be limited to the regular used market, with the +/- 20% restrictions. I think that is the best way to control this.