We have all walked into at least one restaurant or bar that is covered with all kinds of signs as decoration. There is usually an entire wall dedicated to them, and they look really good as decorative pieces. Of course, you cannot just find your everyday signs on these walls, it is usually older vintage ones. There is a huge market for them. A good vintage sign can end being sold for anywhere between a few hundred up to a few thousand dollars.
Old outdated signs are popular because they are limited and not available anymore; so there are only a few owners left for those signs. These signs are also unique because of how they are made. Porcelain signs for instance were very popular during the early 1900s and they then went out of fashion, so the few old porcelain signs that are left now are exclusive because of the way they were made. You will not find porcelain signs being used anymore, so these are a rare item now. People who work in advertising and marketing can appreciate old signs simply because they are related to their field. Looking back at old signs gives people a chance to appreciate how things were done before and how they differ now. So, a lot of people that work with signs, advertising, and/or branding also end up collecting different types of signs because of their interest in their field.
Like we mentioned before, the market to buy signs is niche but ludicrous, so a lot of people go through the buying and reselling process to earn a little extra money and to help other people add to their actual collection. You can find several websites and blogs that are dedicated to the subject of signs and old signs.