No problem.
First is to be as educated as possible about the item including it's place in the market. I love bass so I've been paying close attention since the beginning of my time as a bassist. I do the same with cars but not to the same degree. I don't typically buy very high end gear new because quite often it changes hands and depreciation is your friend. So I can end up with a $6k after some trades for the equivalent of $600. This helps inform you on when to pull the trigger. For example, if you're looking at something that regularly shows up used you can probably wait until you're ready to buy. OTOH if something is as rare as hen's teeth you need to make a quick but still informed decision. That's why I don't miss a lot of deals... I can make that choice. A deal I don't take is usually one I chose not to. I just saw a bass I've kept my eye out on for around ten years but it's not a good time so I passed.
A close second is to make sure you're ready should something you want pops up. Deals don't wait for you. When I saw my future Aries fretless on Craigslist I didn't wait to email, I did it immediately. Then I made an appointment at the seller's earliest convenience. Then I showed up on time and checked it out. Then I made the purchase. Now I own one the best basses I've ever heard or played. Even if you're not prepared to buy you can still stay on top of your research to be as current as possible.
During a couple of periods of times I recognized the resale market on some name brand boutiques was soft and I took advantage of that. That's how I ended up with five Elricks, four Roscoes, four F Basses and four Laklands. Any I sold were below fair market value but not at the low prices I found them. The ones I kept ended up being free.
Which brings me to "I don't ask people why they're selling". Because I don't care. Even if they hated the bass, it doesn't mean I will too. So who cares? I make sure that it suits the purpose I have in mine, typically a potential keeper. Leave sentimentality out of the equation... unless you're buying for sentimental reasons and don't care how it turns out.
I also don't have to like a bass to buy it. Sometimes it's free money waiting to happen. Example: I was going to put a good example here but realized I don't need to put too much of my business here