Been involved in a bit of this lately as Wealdstone were looking to do something similar.
Barnet approached us with regards to Underhill, and asked if we wanted to buy any of the stands, we looked at the big stand behind the goal at the Underground end, and Barnet said it was ours for 100k.. We spoke to four specialist firms who erect and dismantle this kind of thing, to get some pricing.
To dismantle, transport and erect the stand ranged from 180k through to 245k, add in the original 100k and you are talking a serious amount of money.
We then spoke to firms who do the manufacturing and fit out's of brand new spanking stands, and for the same capacity, we were quoted 600k all in. A grant if successful from the football foundation will give you 40% of the cost (you can't apply for grants for used or second hand). So for 360k, we could have a brand new stand, built to spec, with no hassle. It's a no brainer.
West Ham and The Olympic Stadium had seats going free of charge, on the proviso you collect yourself, the cost is astronomical to do this, with no financial help available for non-league clubs, that's why no-one takes them when they are offered.
That said, I believe Exeter have bought Barnet's terracing from behind the goals where the new stand has gone up.