I have a feeling the only reason this works is because the AI is broken and so is starving. In the AI mod i'm using this doesn't work at all.
Explaining it in detail is more extensive than I thought so I just got this screenshot and maybe it'll help.
As you can see I am producing energy from pops and I do have some fallen empire buildings but they're really not needed as I could just sell more of my excess food and consumer goods. Not a giant empire either.
It is also worth mentioning that this doesn't always work all game. Especially early game food is very important for growth and expanding your empire drains all the resources from your producing worlds. But at some point you start to outproduce the cost by so much that you'll have more and more surpluses.
Ive been running test AI games and modding more than ive been playing, but from my understanding, stuff like this is indeed very context depent.
... you can make continous sales on a small scale without the price budging - regardless of other empires market-use.
But when you scale up a bit, it will make a big differance how much of the stuff you are selling that others are buying.... so it matters a lot how big those economies are, how self sufficient they are on these resources - what theyre buying.
The unmodded(/vanilla) AI budgets a lot of energy for market purcheses, and values food irrationally high. It can also stumble on itself and be forced to keep buying food /cgoods , but even if it doesnt, it trades a lot; buys and banks a lot of resources that it has no immediate need for.
Having said all that - on a small galaxy and lower difficulty, the size of a player-empire economy can potentially dominate the galactic economy quite quickly; even if AI keeps buying more than they should their growth stagnates, liniting how much theyre buying. just 4-5 AI + passive market recovery may not be enough to keep prices stable.
... if you alternate between selling two resources tho, you can probably do it - the passive recovery mechanic alone will make a peice recover in 2 years or so.