This will be split into 2 parts: The NPC market, where the players buy items and sell them for the base price seen in
$$price, and the player market. Just a quick reminder, what you can't buy in the NPC market, you can trade in the player market.
Note: Assets listed may have icons next to them. A scale icon indicates that the asset can only be bought from player market. A pink and yellow bag icon indicates that asset can be sold in retail stores.
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 player market is a way for us to trade between corporations and keep the economy and playing field diverse. For example, one might need gold for CPU production but doesn't want to produce gold. You can instead buy gold from another player who is creating excess, say because they have one CPU factory and one gold mine which actually would produce more gold then is needed.
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 $84, 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 $96.
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 produced materials around, so it's not unreasonable to go to $20. You can probably move if you see the prices slowly shifting up, however, so if it gets to $19 it's probably worth going for the $20.
Sometimes it goes to $30 even. Just use your OWN judgment using the prices in the sell and buy offers. The wiki is not a rock-solid "this is the only way to play" guide, but it teaches you what everything does so you can use your brain enough to be able to make a fine decision
Let's say you build a bunch of Television factories, and now you have a bunch of tvs. You then see a $6000 buy 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 marginal profit when you can instead sell every now and then for a larger profit, right?
Alas, these are only baselines. The price changes all the time because of some indirect buffs when adding things into the game (you need more of a rss now) or an indirect nerf (the factory is now outclassed by a better one)