what do you call a person who always thinks they are being used?

Solution 1:

I think the best way to refer to them (politely) is guarded. (Cautious, reserved, suspicious - from various dictionaries.)

Solution 2:

The second definition of cynical describes this attitude:

2 : having or showing the attitude or temper of a cynic: as

a : contemptuously distrustful of human nature and motives <those cynical men who say that democracy cannot be honest and efficient — F. D. Roosevelt>

b : based on or reflecting a belief that human conduct is motivated primarily by self-interest <a cynical ploy to win votes>

_{(Source: Merriam-Webster)}

So, you could say, "He is a cynic," or "He is cynical."