Used trucks for sale is how "trocas usadas de venta" translates from Spanish to English. Trocas can also mean to exchange or barter, but in this phrase the word trocas is translated as trucks.
It would be common to see this phrase when looking at used vehicle classifieds. Another word for trucks is camiones, so an advertisement for used trucks for sale may read as "camiones usados en venta" instead.
Usadas is the word for used, so any advertisement with the word usadas means that the product being offered is pre-owned or used. Trocas may also be used in advertisements to offer an exchange for an item.