Stealing is the wrong word for this, but it is certainly an interesting ethical question. Stealing implies physically taking something from somebody without consent. This is a situation where the person voluntarily agrees to sell at a certain price, but the price is below market value.
Since market value is pretty subjective, I think the cases where it is truly unethical are pretty uncommon. Sale price is partially dependent on seller motivation and need for capital, and willingness to do research. Also, at a garage sale no one can honestly expect to be making "fair market value" since garage sales are designed to be expedient means of selling surplus belongings. One sells things such a way to send them to a new home and get a few bucks for the trouble. If a person really expected to get fair market price for something, they would use eBay or some other method instead of a garage sale... but it's a lot more bother, of course, which eats up some of the marginal return one gets from the better selling method.
If I saw something grossly mispriced, e.g. a pretty, clean 105/1.8 AI-S Nikkor for $10, yes, I probably would say something to the seller. However, if I saw such a lens for $200 I would think that price to be fair enough for the seller. Besides, used goods are always a risk. We've all made poor purchases of used goods.