Irrelevant and not answering any of the claims raised.
The fact that this is what is written does not make it the right way.
When every team who this suggestion relates to find that they can save money by selling the staff at this time, the market will have a storm of staff firing, and then a storm of users buying new staff.
This is a fact and this fact will make it a wrong market definition.
As such, it is needed to be fixed and it is a bug.
That word fact, I don't think it means what you think it means. (Inconceivable!)
First of all, the amount of money to be "saved" is ludicrously small for the teams that would actually have some benefit from this, and would lead to staggering losses for many teams since they must replace the staff in some one to two weeks anyhow, with the signing bonus for the new staff member. If this were something that were exploitable, don't you think in some 20 seasons that it would have, you know, actually happened?
1) Every team replaces its staff once in a (short) while.
They do that for saving few thousands in his contract time.
They will surely will like to save some tens of thousands by that.
2) I guess that the fact that for hundred of years ALL thought that the hearth is flat proves (upon your claim above) that the hearth is flat and not round (as no one exploited that until...)
3) Signing bonus? does not exist.
Firing payment exists, but it also exists in case you just replacing your staff which all does anyhow, once in a (short) while.
Perhaps for (1) and (3) you are forgetting that to get a replacement staff member, you have to bid on them. And while at a point in a staff member's career replacing them becomes a good decision, that is a factor of what his salary is and what the replacement will cost. If you have a level 5 trainer with any reasonable salary, for example, firing him to save two weeks of salary is foolhardy since it'll cost you upwards of 5 times that salary to acquire a similar replacement.
For (2), you can point to data that proves the world is not flat, which directly contradicts the theory. As I have shown you, the rules in this game state one thing and the game is programmed in a manner that bears that out explicitly and correctly. If I say the world is round, and the data shows the world is round, but you think that a toroid-shaped world would give us more land to farm, your belief doesn't mean that there is a bug with the world or the data.
And again, free salary for staff in the offseason can directly lead to exploits allowing people to train with level 7 trainers, a far more realistic scenario with far more imbalancing effects than your mass staff hiring/firing parties could ever have. You know, if you care about the game and balance and all that, think about it for a second or two.