Few things are an absolute.
Most things are simply a generalization.
When "we" answer a question, i don't think any of us expect people to think that what we say happens 100% of the time, its what usually happens.
So, does a big fish sell more than a small example of said fish.
Usually yes, but depends on the fish.
Easy to sell small Oscars and small common Pleco's.
Once they are adults, many LFS do not want them because they CAN'T get rid of them, or they will take them from you for free, and give them away for free.
Depends on the fish.
Small burundi frontosa juvie at LFS about $10.
Small Oscar juvie at LFS, about $5.
Adult burundi frontosa at LFS $50 to $100.
Adult Oscar= Free.
Similar sized cichlids requiring similar sized tanks.
One is more valuable as they get bigger, one is hard to give away as they become adults.