I've been binge watching Antiques Roadshow and so I have been thinking about this issue of values and locations where people buy.
For every discerning yard sale thrift buyer who happens upon a Tiffany watch in a box of junk jewelry, there must be thousands of other yard sale buyers who spend hours and hours combing through crap who never do find a Tiffany watch.
On the other hand, a person wants a genuine Tiffany watch, goes to a reputable seller who can guarantee that this is this is the real thing, they make the transaction, they have what they want with very little time invested.
Yes, it's possible to buy something valuable from a street vendor if you have a keen eye; some of those people one day will be famous. Maybe some already are and are spoofing you (like this), or just currently not placed in a gallery. But, most of the time on the streets you find touristy crap, just as most of the time at yard sales you find people's discarded out of date home goods.
Wealthier people place a value on their time. People who have less money to spend but good taste can spend more time seeking out items with value.