@Duff-Man, can you give some tips (spoiler free if possible) for what to loot, sell or keep?
Did you play Hardcore mode?
I didn't play on hardcore mode, but I am something of a looter, so can certainly try to give some tips
Early game, a lot of things are worth picking up. Look for a high value-to-weight ratio. At the start, anything with a value of 10 caps per unit weight is well worth grabbing.
Grenades are a gold mine... I didn't find them to be very useful in combat, but they weigh only 0.5 and go for around 100 caps a piece. Mines are similarly worth keeping. Weapons are also well worth grabbing... If you have a half-decent repair skill you can usually get more money by using a poor condition weapon to repair a semi-knackered one.
Armour is very rarely worth keeping... If you find some combat armour then that will go for a very large sum, but it's not easy to come by, and early on anyone wearing it will probably spank you anyway. I found cigarettes to be very common and well worth picking up (20 value for 0.5 weight), and early on wine and spirits (whisky / scotch / vodka) are worth picking up (~10 value for 1 weight).
As for other random stuff, ALWAYS pick up the following items:
- Wrench
- Wonderglue
- Duct tape
- Scrap metal
- Scrap electronics.
Similarly, always buy them from vendors if you can. These are the ingredients for a repair kit (3*scap metal, 2*wrench, 2*glue, 1*tape, 1*electronics), and so are absolute gold. Repairing weapons is the best way to earn lots of caps quickly... Weapons are worth a LOT more than they were in fallout 3, especially fully repaired. Even fairly common weapons like the service rifle can be worth a few thousand fully repaired.
On a related note, when you get to level 14 take the "jury rigger" perk, if you can. You will be able to repair alsorts of high-value weapons using bog standard weapons. Repairing a 3000-cap super-sledge using a couple of shovels is an extreme example, but still, you can get a lot of value this way. That said, by level 14 you probably won't be short on caps anyway.
Finally, keep track of which traders have lots of caps, since you will probably drain the caps from the early traders very quickly. I found the gift shop owner in Novac usually has a couple of thousand, and the gun runners always have 8000 caps.
Happy looting!
Edit: Gecko hides and radscorpion glands are well worth keeping to sell. Cazador poison glands are worth a lot, but good luck surviving a cazador swarm at less than level 10!