How would you proceed in the negotiation over the purchase of some land?

Out of the blue a stranger contacted me offering to buy land I own for 75k. I said I needed time to consider the offer, as selling had never crossed my mind. The offer is extremely good and way above my own estimate of the land's value. A day later the person calls again and tells me "name your price and... show more Out of the blue a stranger contacted me offering to buy land I own for 75k.

I said I needed time to consider the offer, as selling had never crossed my mind. The offer is extremely good and way above my own estimate of the land's value.

A day later the person calls again and tells me "name your price and I'll buy your land", then corrects himself to say, "name your price and I'll consider it".

One week later the person increases the offer to 85k, which he says is a final offer. Again, I said I needed time to consider.

So far I haven't commented on the size of the offers nor have I given any indication of what I want

The land has stables but no house, nor does it have planning permission.

*IF* I was selling it with planning permission, the asking price would be higher than what I am being offered (i.e. around 120k to 150k).

Planning permission would be difficult to obtain because of the location, but not impossible. I strongly believe the other person's circumstances or contacts make it likely he will get planning permission - hence the high offer.

If the land had an average sized house, the market value would be around 350k.

I am inclined to sell - but am not sure how to proceed. I have not set any price or expectation myself - any input/advice appreciated.
Update: The land is in a affluent, rural area of the UK. While rural, it is also only 30 minutes from a major city, and has two of the region's most affluent suburbs a couple of miles away. Four or five bed detached houses in the rural area and the nearby suburbs typically cost 300k to 400k.
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