"Bachmann G5 in lined BR black (I've asked about this before, but forgotten the answer).
Pair of Bachmann Ivatt 2MT 2-6-0s, one in BR black, one in BR green.
Bachmann Derby Lightweight DMU.
Dapol Class 121 'Bubble-car' single unit in BR green.
Hornby Class O8 in BR green, 'expertly weathered by TMC'."
I think that these items would reach within 10-25% of the retailer prices on eBay. If passed on to a second hand "specialist" significantly less possibly as much as 50% of RRP.
Ultimately it's how quickly an individual wants to off load stock.
The G5 should be in more demand than others. Only because it's newer and more rare.
TMC weathered rolling stock doesn't necessarily increase or decrease value.
I wouldn't want to be any more specific.