Is there a basic item pricing guide somewhere thats shares what item costs are? How does everyone seem to know how much something is worth, but noone can tell me how much something is worth?

There is none, prices change constantly.

Use the search function in trading forums to see how much last person sold the same item for and use this.

Basically this.

Side note - don't trust offers made through pms, especially if you don't know the value of something. Some are legit, most are people trying to underpay without being named and shamed.