This will be split into 2 parts: The NPC market, where the players buy items and sell them for the base price seen in, and the player market. Just a quick reminder, what you can't buy in the NPC market, you can trade in the player market.
This section is a stub. You can help out by adding more information here.
The NPC Market is the original market for all your essential needs ever since Update 1.0.0. You can buy and sell there for the prices in the assets page.
Ever since Update 1.5.0, the market is a way for us to trade between players and keep the economy and playing field diverse. For example, one might need gold for CPU production but doesn't want to produce gold. He can instead buy gold from another player who is creating excess gold, say because he has one CPU factory only but one gold mine actually would actually too much.
A good rule of thumb is to buy at about 1.4x for most products, 1.6x for VERY useful non-end products, and 1.2x for not-so-useful non-end products(except for <$1 assets which are mostly $3, because clearly selling for low makes it worthless to do so). End products sell at a very varied range which you just have to test out. If nobody else is selling, supply and demand should tell you to sell for more. Likewise, a massively overproduced item is NOT worth selling for the prices above. This ranges a span of about 1.2x extra on both sides.
For example, gold can be sold for $72, as it has few use cases in the current self-producing end game structure (only a CPU factory ever needs them in the optimal end game strategy as of Update 1.7.2. However, if only a few people are selling gold, it's not unreasonable to sell for $84.
Imagine now you are a pure energy company. There is a currently relatively huge demand for research, maybe because somebody invited tens or maybe even hundreds of people. Energy is really useful and normally sells for about $16. In this case, however, $18 might be a good call, but energy is one of the less fluid prices around, so it's not unreasonable to stay at $16. You can probably move if you see the prices slowly shifting up, however, so if it gets to $17 it's probably worth going for the $18.
Let's say you build a bunch of Television factories, and you see a $6000 offer. Clearly you should sell.
1. Outbid the player with a $6000.01 offer so you're first when people sell to you.
2. Go for something like $5,000 even if you have to wait.
3. Go for exactly $6000, this makes it so
$$msell tv all also hits you when a player mass-sells tv.
The third option is hardly ever the correct play and is niche. That leaves the 1st option and the 2nd option. The 1st option is really only if your retail stores sell televisions for a WHOPPING 10k+ and you're running out of TVs fast, while the 2nd option is the better case, based on 1.2-1.4x(optimal buying price based on several factors). You don't wanna sell when it's only a small margin when you can instead sell every now and then for a larger profit, right?