First of all, I really think this game has the potential to be great. However it is in dire need of quite a lot of polish and refinement. In its current state I would have a difficult time recommending it, despite its great idea.

The most immediate, major problem is the UI is horrendously unresponsive, ugly, and frustrating to use. In games with several other players it is very common to have a 5 second or longer delay in order to perform any in-game action, which is practically unplayable as a result of the UI being so unresponsive. This really does need to be addressed, and quickly. Most of the actions in this game are performed using menus and clicking buttons, and it is critical that those menus are responsive.

Gameplay

Looking past the problems with the appearance and the UI's responsiveness and addressing the game's mechanics in detail, there is also a substantial amount of room for improvement. 

The biggest gameplay issue is that players don't have enough reasons to buy anything, but have every reason to need cash. This creates an economy where everybody is selling, and nobody is buying, cratering prices. Players are too productive, everyone is selling, and nobody is buying. By the late game you basically just sit there shipping goods into space, with practically no player interaction or strategic play at all. 

Additional mechanics, buildings, etc. need to be implemented that give players productive ways to spend resources. If the prices of inputs are low, you could purchase commodities and turn a profit. Market manipulation is only interesting if you can disrupt someone's economy. As it now stands "market manipulation" basically serves to pump up depressed prices for a commodity you have a large amount of, so you can sell for a slightly higher price for a short time.

The game needs a reason for players to consistently buy, so there is an actual market to sell those resources to other players.

Trade

Space trade is a very boring mechanic as implemented now. Shipping goods into space just creates cash with no player interaction, basically ignoring the market, and lets you just start buying stock with that money. Player interaction is the hallmark of any strategy game, and in the late game the amount of player interaction should be at its highest, not a non-factor as you manufacture resources, ship them into space, and buy people out.

Interstellar trade should be a centerpiece of a game called "Offworld Trading Company" rather than by far its least interesting mechanic.

Buying Stock

The concept of buying stock to win the game is cool. However, as currently implemented buying stock has almost no causal effect on the game until you completely buy someone out. You just need to pay a certain specified number of dollars to eliminate a player, and it even quotes you a specific number.

What should happen is that the price of a player's stock is based on the price that players are willing to pay to purchase that stock, and not simply a flat, predictable cost. It should be possible to buy any stock, not just stock that has not yet been purchased. If Player A buys Player B's stock, player C should be able to buy B's stock from A.

Instead of individually negotiating trades, this should be done with a simplified, but semi-realistic put/call system. The way this would work is each player has an inventory of stock for each player. For each type of stock, each player can specify any strike price that player considers acceptable. If there is a player prepared to buy at your specified price, then that player buys the stock from you. If there are several players stating a price higher than your specified price, then the highest price wins.

One effect of this is that completely acquiring another player's corporation would involve some uncertainty as to exactly how much cash you will need, especially if that stock is split among several players, one or more of which might be relatively unwilling to sell.

Owning Stock

Owning stock should also have an effect. The ability to sell stock to other players is one way this might be important, since you would be able to buy stock expecting that player's stock price to rise, and then sell that stock to another player.

I also think that players should be required to pay dividends, so owning the stock of other players' corporations can be profitable by itself. The stock of profitable players' corporations would be highly desirable by other players wanting a share of the dividends, which will cause its stock price to rise because players are prepared to pay more to obtain shares of that corporation.

Credit and Debt

The economy on the planet would be made much more interesting by having money be a commodity, much as it is in the real world. Availability of credit should depend on other players' willingness to lend.

Like the stock system, this would work by each player declaring an interest rate at which they are offering loans. Whoever has cash and the lowest rate at the time will be lending money to players seeking credit at that time. If everyone is broke then the game will look very different than if everyone is keeping large cash reserves.

Conclusion

Overall I think Offworld Trading Company is eventually going to be an excellent game. But right now it is lacking in polish and could use some more strategic interaction rather than build order optimization followed by non-interactive space shipments financing equally uninteractive stock purchases.