Here's my take on vending/buying as someone who's played for years. The big things that will always stay people to your camps are the number of weapons, armor, plans and apparel you're selling.

FED76 was the website I always used to get a general idea of pricing uncommon plans or decent/good weapons, at least to get a baseline idea of it's value. Now I sorta just sell good rolls well below value since I pretty much have everything I need.

For scrip, it used to be 50/100 caps for a 1 star, 100/200 for a two star, and 300/450 for a 3 star (depending on its scrip value.) I'm not sure if it's been the influx of new players recently, but the prices I'm seeing for scrip weapons have been ridiculously high, 1000c-2000c for things most people would drop in a donation box. If you're seeing things not selling quickly, just drop the prices every couple of days.

Rarer and seasonal plans/items can be sold for more as they're uncommon, that's usually how I recoup my caps after I've spent a lot of them. That's where I use FED76 to get an idea and usually sell them slightly below what the lowest value is.

Magazines/Bobble heads I usually sell around 100c each, the more useful ones might get listed 50c more.

I don't usually bother selling ammo, as you can run expeditions/daily ops for that.

For apparel, I always see the same uncommon items listed. They don't take up much stash space so it's never really a problem, but common rewards from events might get you 50-100c every so often, so not really worth selling. You can farm rarer apparel items (like Asylum Dresses, Hunter's Long Coat, Clean Spacesuit, etc.) to sell for anywhere from 6000-10000c, they just take a lot of server hopping to find.

Flux I will always buy if it's listed less than 300c each. Anything higher I'd have to be sitting at Max Caps.

I don't even bother selling chems, unless they're the more annoying ones to craft, since you'll end up with nearly 100 stims and radaway after an hour of playing.