Time does have some influence on price for tents. Unfortunately, fabric 'ages' both with use (UV etc) and regardless of use. I've seen lots of tents where the PU flysheets go 'sticky' over time as the proofing breaks down with age, often when they've been stored.
I'm sure you're being truthful over the amount of use, though a lot of people claim 'unused' or 'minimal use' and when the item arrives, it's clearly been ragged. This is why s/h prices are often reduced due to age as you can't neccesarily believe what you are being told by the seller.
However with Hilleberg, you've got the RR of tents and if they really cost £450 (rather than being RRP £450 and being available for a lot less) then there should be no reason why you shouldn't get £250 for it assuming 3 nights use and unmarked/unblemished/undamaged.