All content is accurate for 3.67 Frontiers.
Crashing a market involves selling large amounts of any material or trade commodity and buying it back at the prices reduced up to 40%. The best commodity to use for crashing a market is by far Cobalt. At ~200 units per item it is valuable enough, and every Space station sells it. This is important, because you can only buy back the commodity that a current terminal sells. The big advantage of Cobalt is that you can mine sufficient amount to start market crashing in caves on your starting planet. 2,000 is the minimum amount recommended, although you will have some gains from as little as 1,000. When the Cobalt amount at your disposal increases to 10,000 (one full stack), you will get ~1.6M Units by selling this stack and buying it back.
After the Star system market is crashed, you may be able to buy additional amounts of Cobalt from NPCs landing in the Space station. By quickly jumping between Space stations and crashing the Cobalt market it is possible to earn up to 4 billion Units in less than 24 hours of game time, and completely upgrade your Exosuit storage at the same time. Adding other high value materials (Gold, Silver, Platinum, Ionised Cobalt, and Chlorine) to your inventory can further increase the profits, but these can only be sold at the station that already sell them.
Wiring Looms were also a very popular market crashing commodity, until in Synthesis two prices were introduced: 25,000 Units for selling, and 50,000 Units.png for buying, which requires a very large amount of them to crash the market to a profitable level (300-400), which in turn requires at least 15M initial investment, and at least 2 purchases from tier 3 economies. Still, it is a useful commodity in Survival and Permadeath game modes where material stacks are limited to 250/500, so a full stack of Cobalt can only bring ~80,000 Units vs ~7.5M Units for a stack of Wiring Looms.