We don't do valuations here - sorry. There are so many variables when it comes to buying and selling lamps and lanterns that it's just not possible to predict sale price. Ask the seller if it works - it won't - and use that to lower the offer price. If he says it does, then ask him to show you how to light it so you can see it running. A pound to a pinch of snuff he won't know. Ask him if spare parts (such as the glass globe) are available and where from - he very probably won't know that either. All this shows the lamp in a bad light and will lower its value in his eyes. It's all worth a try and you might get a bargain. Check completed on-line auctions for sale prices. One of these sold on eBay UK last night. At the end of the day, just offer an amount up to what you are comfortable with spending for it...
Ooh, dunno - I reckon there's a good couple of days work there, I would have thought. You must be a quicker fettler than me Claus! Would be good when finished, though...