Over the years I've developed a pretty good sense of what's a good deal and what's not. Pretty regularly I see basses and amps on the local FaceBook Marketplace that would definitely be candidates for buying and then selling for a profit (maybe 15-25%) depending on the item. For example, there is an Stingray 5 that just popped up in what looks to be great condition for $1.2k. If it's as good as it looks, it might go for $1.5k here on TB.
But...that's before shipping and then there's the time invested in going and looking (and probably often, passing due to condition or other issues) at them and the cost and effort of finding packing materials, etc.
For me, I keep an eye out for things on my wish list and try to get a good deal on those. If they don't work out, I should come out a little ahead or at least break even. I've pretty much kept most of what I've bought, but for the things that weren't a good fit, I'm a little ahead. Not trying to buy and sell for profit, but not losing $$ keeps my wife happy!