There are, of course, many online second-hand markets available, eBay, Gumtree etc. This idea is inspired mostly by Facebook Marketplace though and would probably work best as an add-on to their system rather than a standalone app.
Facebook Marketplace allows you to search for products to buy within a certain radius of your location. Because Facebook has such a huge database to draw from, this has been very successful. In South Africa, at least, Facebook Marketplace has become the first place to look when you are trying to buy something preowned.
The idea is to create a similar app, or preferably, just an add-on to Facebook Marketplace that keeps searching for the items you want even when you aren't actively scrolling through the listings and notifying you when a match comes up in your live location.
This will allow you to see options you would have normally missed when you are travelling or simply find yourself in another part of town. You would save the specifications of your search to be as detailed as you'd like, for instance, if you are looking for a guitar, you might choose to be notified anytime you are near a listing for "musical instrument", or near one for "guitar", "Fender" or "Fender Mustang 1964" depending on how specifically you are looking for something.
This would benefit collectors more, as they won't mind waiting for a very specific item to pop up, that might not be as readily available.
One could argue that you could simply set your search radius on Facebook Marketplace to its maximum and make sure you check it every time you are in a new location, but I think an app with this capability would surprise you by finding what you want in places where you weren't expecting it, or at times when you might not have been thinking about looking for it.
Sellers' adds would travel along with them, so you might even find a product because someone who has one for sale is visiting your location. Which would add to your chances of finding what you are looking for.
Once matched with a seller in your area, you would receive only his/her contact details, making this no more of a security threat than any other advertisement online.