What your Barbie dolls are worth depends on many things. They can range in worth from just a few dollars to several thousand dollars. The 1959 Barbie in mint condition has sold for as much as $8,000! You can find more information here: http://www.essortment.com/all/barbiedollvalu_rrms.htm
Great question. Value is relative. In order to find out how much the guitar is worth, you have to find out how much people are willing to spend on similar items. This could give you an idea of how much you can expect to get from the sale of your item.