At the moment there aren't many aircraft that SELL for over $200k. Many are listed at high prices, but I know of at least 2 aircraft that sold in the last few months at half the initial asking price.
I would add that unequal syndicates (ie one person owns 75% and another 25%) is a recipe for disaster. The minor owner has no vote and gets lumped with the bills. Either syndicate the aircraft properly or not at all.
Beyond syndication, none of the options to earn money from aircraft are very good. If the a/c has had $100k spent on it it must have been either a) to fix an old aircraft (eg old Bo) with problems, b) a glass panel or c) engines on a twin. I guess it could be an engine & some avioinics on a single. If it was worth $100k pre upgrade, then its a serious aircraft - so it is unlikely to be a training aircraft or one you'd want to trash with training. This leaves private hire or charter. Charter is starting to get arduous for maintenance and these days needs to be one of a handfull of aircraft 210 / B58 / PA31, etc. Private hire might be an option, but the market is pretty small - especially if you put flight experience & currency requirements on the aeroplane to try and protect it.
The best syndicates I've been part of have had no agreements. Its just mates who respect & trust each other dealing with honour.
Once you sell a share, its the new shareholders problem to sell it. If he moves interstate and can't sell his share then he still must pay the share of the fixed costs. If he sells it at a loss to get out quickly, its his loss. If he sells at a profit - good luck to him. Once the original owner sells a share its none of his business.
Each shareholder should have a power of veto to stop one shareholder selling a share to someone unsuitable, although this would commonly then require that shareholder that rejected the prospective buyer to then buy the share at the proposed price - maybe not the first time, but at least the second or third.
There could also be a "shotgun" clause where if one owner offers to buy the other out at a price and it is rejected, then the other is required to buy the initial shareholder out at the proposed price. This means that if shareholder A offers shareholder B $10,000 for the share and B rejects it. Shareholder B then has the ability to buy out shareholder A for $10k