Sell Price Multiplier
Sell Price Multiplier is a stat: A multiplier on the price at which you can sell items.
An item's multiplier is solely determined on the type of item:
The default value for all other items - including materials, food, apparel, etc. - is 1 (100% market value). These multipliers only apply for selling, not buying.
The multipliers make it more difficult to farm raids for profit, because weapons dropped by dead enemies are not valuable in trade. It also means that crafting weapons in your colony only makes sense for personal use, and not for selling. The game encourages creating art for sale, as it is about 57% more profitable than regular constructed items.
Trading comes with additional modifiers, a base of 60% when selling and 140% when buying, documented in Trade Price Improvement. These modifiers are unrelated to the Sell Price Multiplier, and applied independently.