The word “used” can have different meanings, so here are some synonyms based on those different senses and an analysis of each:
Worn: When “used” means something has been in use to the point of showing signs of wear, “worn” is a good synonym. For example, “a worn pair of shoes”. “Worn” emphasizes the physical deterioration or damage that comes from use.
Second-hand: This is often used when referring to items that have been owned by someone else before. “a second-hand book”. It implies that the item has had a previous owner and is not brand new.
Pre-owned: Similar to “second-hand”, this term is a bit more formal and is often used in advertising for used goods. “a pre-owned car”. It gives a sense of respectability to the previously owned item.
Accustomed: When “used” means being accustomed to something, “accustomed” is a suitable synonym. For example, “He is accustomed to getting up early.” “Accustomed” emphasizes a state of being familiar with and comfortable with a particular situation or behavior.
Familiar with: This also conveys the sense of being used to something in the sense of having experience with it. “She is familiar with the new software.” It implies knowledge and experience gained through repeated exposure.
Habitual: If “used” implies a habitual action, “habitual” can be used. For example, “He has a habitual way of doing things.” “Habitual” emphasizes the regularity and predictability of an action that has become a habit.