I have the following NOS parts in original GM boxes that I am trying determine how much they are worth:
GM left RS fender
GM right RS fender
GM left door skin
GM right door skin
GM left convertible quarter panel
GM right convertible quarter panel
GM front upper valance panel
GM front lower valance panel

I also have the rear tail panel but it is not in its original box so I'm not sure it is NOS.

Are these more valuable together or separate?

Look on ebay for completed auctions for the prices.

Fenders- $700 to $1000 ea
Door Skins- $300 ea
Quarter Panel $1000 to $1500 ea
Upper Valance $300 to $500 ea
Lower Valance $300 to $500 ea
Tail Panel $300 to $500ea

The economy is bad right now and these prices were when it was good. Good Luck

You will make more money selling them individually and will be easier to sell. Good luck on shipping cost since they have about doubled now.

Keep in mind that your net from Ebay sales is not what the buyer pays. Between Ebay fees and Paypal you loose quite a bit. I was able to sell my NOS 69 coupe quarters here and got my asking price of $2800. I figured that's what I would have netted from a $3000 Ebay sale.

Jeff

Regarding your tail panel. Look at the back of the panel. On the taillamp mounting brackets that are spot welded to the tail panel there should be some numbers and letters stamped into the metal. Only GM tailpanels had those. If they are there you have a NOS panel. If not, it's repo.