I do quite a lot of this at all price levels, so here's what I've found:
1. Decide what said instrument's use is. If its a player, skip the rest of this post. If you're buying it as a potential investment piece, keep reading.
2. The market for any optional/luxury good is very fickle. Absolutely know particulars, timing, and real selling prices...or just go burn your cash in the trash heap now. Become a student of market demand and actual cash values.
3. Be an unemotional buyer. Set parameters for condition, price and variations in color/finish/options etc. stay within your parameters. Some of your very best deals will be those that you walk away from. However (and this is important!!)...don't dick around when you see a pice that fits the puzzle and its from a trusted seller. You DO have to have inventory to make money. And a good relationship and reputation can go a long way when it comes time to buy OR sell. Keep your word, even if it hurts!!
4. Here's where everyone falls on their face: BE A GOOD SALES PERSON and MARKETER. The old saying that something is so good "...it just sells its self" are the famous last words of a guy on the edge of bankruptcy. Learn to speak intelligently about each bass you have, it's features and benefits, and how those advantages may be advantageous to the buyer.
5. Every single guitar or bass has the potential of profit if purchased correctly, marketed properly, and sold at the right time.
I buy Boutique basses all the time and make money on them. How? I buy smart (for me), and I actively find the right buyers and sell smart (for both of us!!). I make very good money in used Ibanez basses and guitars. How? I seek out the rarer models. I look for specific features on all guitars and basses that translate into eye candy and perceived value. It's work. It's NOT my full time job, but its fun and as steady of an income as I decide for it to be by how much work I put into it.
So what brands do I favor right now?
Duesenberg guitars will be valuable one day. Very unique.
Red Rocket guitars will be VERY valuable one day. Look for used guitars by them in the coming years.
Later model Roscoe and Mike Lull basses, bought used, can work out very well. Tops and fancy boards go a long way with any basses. Sure MIA Fenders are SAFE...but actual profit, cost adjusted for inflation (if you've kept it out away in your closet) can be a bit flat. Ric's are good, generally. They key is UNIQUE and letting someone else take the depreciative hit.
As for Warwick basses, they are beautiful and amazing instruments, but frankly, I relegate them to "eBay basses" because there are literally hundreds for sale at any moment online. Too much price pressure and competition.
Big scores? Used Alembics bought well with Omegas on the body in dark woods. Status Graphite. PRS basses, when you can find stunning examples, are a ticket to print cash if you're willing to negotiate a good price of purchase and then find the right buyer.