Why do caps go missing sometimes when your barter with people?

For example, if I have 1100 caps, and I sell 700 caps of stuff to someone I would expect my caps to increase to 1800 caps, but they don't. Sometimes they only increase to 1200 or 1300 etc.

Does anyone know why this happens?

You sell low and buy high based on your charisma and cap collector level. And it is capped at 8 charisma and level 2 cap collector so you always sell for less than you buy.

What I mean is I am not getting the amount of caps that I should. If I am selling something, and it says I am getting 700 caps, then when I choose 'Accept', its not giving me the 700 caps, only 100 or 200. You haven't noticed whilst playing that when you sell stuff you don't get you the caps you are owed?

The system is setup so caps are not deducted immediately for purchases either. So when you buy something, and sell something else, the difference in price is what is transferred. Buy 400 caps worth of ammo, nothing is subtracted, sell 700 caps worth of junk, nothing is added. Hit accept, you receive 300 caps, the difference....

Watch out for pickpockets in diamond city. They're worse than those synths that Myrna worries about.

If the vendor only has 200 caps and you sell him/her 700 worth of goods you will only receive 200 caps.

Lets say I have 1000 caps. The vendor has 400 caps.

I buy 700 caps worth of ammunition.

I now have 300 caps and the vendor has 1100 caps.

I sell Purified Water to the vendor worth 1100 caps.

I now have 700 caps. I should have 1400 caps?

That's communism at work there my friend!