If you have ever signed into FB from home, they know that machine / ip / whatever fingerprint is you. Even after you sign out, its still out. Even if you are in ingocnito mode, its still probably you. Then you sign in to FB at work, and now the same thing happens. They may not know the specific computer, but they know the network you are connected to IP wise.
This happens across many different ad networks, and they are all connected to sell ads targeting the people something wants. If you leave a potential purchase not bough, and sometimes even when you do buy it, ads are going to make it back to 'you'. They may not know 100% that it is you, but they can cast a net to try and find you. And for something like a missed sale, its worth the cost.
Note it may not be targeting you specifically. It may just be closer to: Someone in this VERY specific demographic looked for this instrument, oh hey here is someone else that meets that same demographic on this other ad network. Lets try them.
The amount of tracking on the internet nowadays is absolutely crazy. Visit practically any website, and all the major players are going to know about it and add it to their profile of you.