On a recent edition of Flog It, there was a Hornby Dublo 3 rail set, with extra track and rolling stock, all boxed, which went for £50. In my local auction house this week, there was an A1, two blood & custard coaches and a guards van (!), with some track, 3 points, "Woking" style signal box, level crossing but no boxes, estimate £100 -£150.
I know 3 rail is getting rarer, as the collectors buy it and squirrel it away, but what would you pay for older N gauge?
It really depends a lot on the model, its rarity and if anyone actually cares about it being "rare".
So you can expect to pay £150+ for a class 73 in Pullman livery, or £40 for a Hornby Lyddle End East station building and insane amounts for Kato extra Eurostar coaches but peanuts for a lot of much older stuff.
Some of the old long discontinued kits go for £50+ as well, especially the Peco/Wills loco kits and the Hughes ones.
Not much of the British RTR is worth a lot though - or only if its boxed. A mint boxed early Farish set can be worth a bit if the right one but the bits on their own don't go for much at all usually.
There are some real rarities out there like the Heritage N class 85 or the Silver Fox 66 but it doesn't always mean they are worth anything cos nobody wants one!
Alan
At auction, any item is "worth" 1p more than the second richest/foolish person is willing to pay...