I have no difficulty believing that buying and selling guitars and basses as a serious investor, as in putting significant money into the enterprise in an ongoing way, is basically a full time job. One reason I don't own any, though I have some money to invest.
As an "investment," I would recommend almost anything but vintage instruments.
I mean, if you run across a lovely all-original pre-CBS stack-knob Jazz at a rural estate sale for $300, by all means get it and store it well...but if you are going to pay market price for a good, 100% professionally-authenticated piece, it's going to be a
long time before you double your money.
Don't "invest" in sexy stuff that you already know is in a corrupt and overheated market just because it's exciting to you.
I'm a huge advocate of people buying and selling stuff to generate "black" income and develop expertise, but it doesn't make sense to paddle around in shark-infested waters where even the sharks are eating each other.
Investment is about making money, period. Doesn't matter with what as long as it makes sense.
My next-door neighbor is a really neat guy. He's from Niger and had high hopes of doing custom woodworking, his "passion." However, he accidentally discovered that he can make more money flipping specific power tools about which he's developed expertise (notably Delta bandsaws) than he'll ever make on "sexy" woodworking. He'll buy a bandsaw on CraigsList, tune it up, maybe replace the blade or tires from his eBay suppliers and put it back on CraigsList a few days later and double his money on it, and all he's done is put a couple of leisurely hours into spiffing it up after work. He knows the stock and he knows his market -- perfectly.
Exciting? Well, maybe not, but it's fun for him and his profits are predictable and nearly approaching what he makes in his day job.
If you want to make money with instruments, it's
far easier to do by reselling modern gear on which you've gotten
very good deals. You'll make more doing this -- if you know your stuff -- than you will "investing" in dubious vintage instruments.