i've been looking at some price guides trying to get a feel for what some of the items cost, and i have seen on ALOT of items listed, where it says item X is Worth 0.5 def pot.. Now when going for an item like this, would the common thing to do be use a diff kind of pot and /or try to get the seller to throw in something else worth another 0.5 def pot, or how does one sell a half of a pot if thats the case..

half a def pot is mostly an att pot

Look at other potions. 4 SPD = 1 DEF, so 2 SPD = 0.5 DEF. 1 ATT = 0.5 DEF. 1 WIS = 0.5 DEF. 1 DEX = 0.33 DEF, though you'll sometimes see people offering 4 DEX for 1 DEF, or 1 DEF for 4 DEX.
And again, the price guide isn't the law. An item is worth as much as someone is willing to pay for it.