Why am I collecting items when it appears that there is no place to sell them, besides the vending machines and they only buy certain items.

Thanks

Those vending machines will except a whole lot if you find the right codes for them.

Because the holo vendors carry lots and lots of precious, weightless pre-war money that will make you vastly, mind-bogglingly rich in the Mojave.
Just trade your unwanted items for pre-war money. When you get the opportunity to do the same with Sierra Madre chips, I recommend doing it.
Hell with that heavy gold. The real treasure in the SM (heh) is weightless.

What Al D said, use pre-war money.

And any money you don't trade for caps has a second use- ever notice there's not a single roll of toilet paper in the entire wasteland?

This is why pre-war money is always in demand.

I'm confused as well. I came across a vendor and he had EC Packs. I tried to buy them. Couldn't. He had no caps. I had no caps. I tried to buy stuff with pre-war money. No luck. I tried to kill him. Again, no luck. What is the point of the vendors?

I use NCR 100$ bills, myself. It's the only thing NCR money's good for.

Barter. Give the vendor some stuff, take some stuff back.
It's a pain without the running total, but you'll get the hang of it.

I'll try that, I'll have to go back to him and see if I can buy those EC packs from him using barter. Thanks.

You're welcome. Don't forget that if you lose patience with the bartering you can always trade at a loss (just to get rid of stuff).
And stock up on pre-war money. Better than gold.

You trade the stuff you want to be rid of to holo-vendors for pre-war money packets. To really make a killing though, cash in all your Sierra Madre chips for pre-war money before you leave. 1 SM chip = 1 Pre War money = 10-12 caps back in the Mojave. If you cash in 10-12,000 SM chips...which you should have if you broke the bank...for Pre War money, you'll get about 100,000 caps for them back in the Mojave. Don't forget to gamble until they ban you....if you don't get the voucher to cash in at the Casino you don't get them back in the Mojave.