If my eyes don't decieve me, that's a short nosed C10 (C10 as opposed to GC10) with a four pot G series engine in it. If so then it's not as desirable as the regular six cylinder cars. The proportions of the short nose body in coupe form are not good at all! I'd love to have a short nosed saloon or estate though.
It's impossible to value as there's no precident for this model here. In this case, if it sells for £15k then it's clearly worth that to whoever buys it. personally, I wouldn't pay that for it, but then I wouldn't particularly want it as it's not a model that apeals to me much. I would suspect it'd be possible to get one in similar shape from Japan for less but a) you need the contacts and b) it's a bit of a gamble buying blind whereas this one you can view. It'll be interesting to see if it makes the money he's asking though.