A little corrosion can build up if gear is not stored in an indoor, air tight environment, especially if it is anywhere near salt water. I live about eight miles from the beach, but the air is still somewhat salty, so anything outside is prone to some corrosion over time, especially external corrosion.
If the gear you bought has more than just slight surface corrosion, or more than a little amount of internal corrosion, then it has probably been used at some point in time. With this being said, it is difficult to tell unless you have a trained tech disassemble it completely to give it a good going over. The trained eye can detect use/non-use/wear/corrosion of parts, but it can be difficult if you are not trained what to look for. If the corrosion you see is that obvious to you and you are not a tech, then there is a chance that it may have been used, even if only a little. You also have to factor in how well you know the seller, how much you trust her, and how much you paid for the equipment.
I have bought some gear in the past that was advertised as "unused" or "never wet," but upon close inspection I became suspicious. I believe some divers must use something once or twice, don't like it for some reason, and then rinse it and store it somewhere until they can get rid of it. Some things can be stored for so long that some people may not remember if they tried it or not (unlikely, but possible). Now I ask if the gear is "new" or "as new."
A good soaking in a weak solution of white vinegar and warm water will rid most metals of light to moderate corrosion due to the acid in the vinegar. However, this should not be used on silicone, rubber, or other non-metal parts (some may recommend a very weak mixture for some non-metal parts using a shorter soaking time). A good stiff toothbrush is very helpful for getting into small spaces, and a set of dental picks is good for gently scraping tiny areas like metal threads.
As corrosion is removed, just remember that most regulator 1st stages and some 2nd stage components are chrome-plated brass. When the corrosion is removed, you will be able to see the underlying brass which may not be pretty, but is still very strong in most cases, especially for the larger parts.
Hope this helps!
