Personally, I think the fact that the MC didn't have to monitor the slave market and buy her was just a gameplay and story segregation mechanic, like Lin locking herself in her room while *also* loitering around in the lounge.
The story is that Maria gets home in a reasonable time frame, so having to wait a month to collect the funds breaks immersion worse than having Ellen do all the work. There may be other ways around this, but this seems like the easiest one. It should also be noted that the final Maria event is forced on you, and you cannot delay it, so having to find the money to buy Maria may seem like the game is punishing the player for no reason, if they are not in a good financial spot at the time.
Of course, we do have a ridiculously rich man on speed dial, and it would probably feel somewhat more empowering for the MC is we could call and ask him for help - as his potential son-in-law, or just because a friiend of his dauhter is now being sold as a slave.
Granted it wouldn't be us doing the work, but at least it would be us finding the solution.
It's also a cheap fix, since it doesn't really require any additional images, and it can even work to make Cornwell more of a villain (Trenero offers him money to forcibly take Maria from a bad master, Cornwell refuses because he actually thinks half-elves are worse than animals and should have no rights, so we have to wait uuntil maria is actually sold by her new mistress).
I hope Runey will consider it.