Characters can usually buy and sell items only during downtime. An item can typically be sold for only half its
Price, though art objects, gems, and raw materials can be sold for their full Price. The PCs' ability to sell items plays a big part in their ability to equip themselves how they want. However, it may not make sense for the story that PCs can find a buyer for every item, especially if they're selling multiple copies of the same thing. In a megacity, port of call, or major trade hub a PC should generally be able to sell whatever they want whenever they want thanks to convenient infosphere shopping, all-hours markets, and high population of buyers. They might be unable to sell items that wouldn't be in demand or have to take a lower percentage in remote places that have low populations or lack wealth.
If you don't want to deal with that level of detail, you can choose to make selling items more abstract, allowing the PCs to sell anything for half Price essentially at any time. Since this makes it far easier for PCs to outfit themselves how they want, they might be more powerful.