They're probably selling it as a potential development site, not as an agricultural shed, and if that's the case the valuation of £20-£40k is irrelevant.
The shed will have development potential under permitted development (class Q), regardless of whether actual planning permission is in place at present. Permission will still have to be applied for, but as long as it meets the permitted development requirements, it will be granted and will cost next to nothing to apply for. Imagine a scenario where you buy it at the higher end of your valuation, develop it into a 4 bed house on a decent plot in the middle of the countryside under class Q, then sell it for £400k - the buyer is hoping to take a bite of the cherry before selling, without taking any of the risk themselves.
It sounds like this is a dilapidated 'modern' shed rather than a traditional barn, but lots of these types of buildings have been converted into houses since the rules on permitted development changed.
If the site was a building plot with planning permission for a house, would you be surprised at the £200k valuation? Obviously it's impossible to tell for sure without seeing a link and knowing more context (and I realised that would be really outing), but it sounds like that's how the vendor is viewing it.
Pay particular attention to covenants. You might be liable to pay a percentage of the uplift in value on the grant of planning permission.
I'm a rural surveyor and have worked for landed estates for years. We're always flogging this sort of stuff and round here (Cornwall) people will pay silly money for these kinds of places, because they're like gold dust.
As to whether the estate agent is employing tactics to generate interest, it's hard to say but I wouldn't usually expect to put a closing date on a property that's not generating any interest. Have they asked for best and final offers?
You would be buying your privacy and you need to consider how much you don't want someone else to buy it and have control over 'next door'. We live down a lane in the middle of nowhere and have one neighbour (lovely) next door. She's older and has no family around here so will likely want to sell at some point. Thankfully she has kindly offered us first refusal when the time comes. We don't particularly want the property because it's a tiny, dilapidated, dark 2 bed cottage with a massive garden and we already have loads of outside space to maintain, but we will have to find the £250k to buy it and then the additional £75k to renovate it because it will probably be our best and only chance to get control of it. Luckily she's fit as a fiddle and bright as a button, so I'm hoping we've got a while to save up!
Good luck with it. It's hard to know what to do but you can only offer what you can afford and what it's worth to you.