take shotgun for example the cheap one. it sells for 8caps but if i where to buy it it's value is 115caps does that sound right or am i missing somthing? all weapons an armor are selling to vendor for way cheaper trhen it would cost me if i where buying

Yes, that's normal. Investing into the barter perk (1st one under charisma) and finding magazine issues of "tales of a junktown jerky vendor" will improve buy/sell rates. Eventually, you can get full value for items.

Max out the barter perk and start investing into shops to increase their max cap amount. You're gonna want to get full value for a lot of the stuff you offload.

Yeah the vendors rip you off really bad in FO4 (unless you have high Charisma & Merchant perk). They give you like 1/8 of what the item is worth. One thing I did notice also was that merchants don't really care about Alcohol. I know that sounds funny but I used to be able to sell alcohol to merchants for good chunk of Caps in the previous FO games but now they don't care about Alcohol at all.

Yup...a merchant wants to buy something as cheaply as they can and sell it for as much as they can. They make their living on the difference between those two numbers. Barter skill in prior games...and in FO4 the appropriate perks from the menu or "Tales of a Junkyard Jerky Vendor" magazines...represent the PC's ability to convince the vendor to take less of a profit than they normally would. Take the right perks, find and read those magazines, and wear clothes or do chems that buff your CHA to get the best prices from vendors.

Caps are not pennies and what you have is usually worth what you can get for it in whatever economy you live in.