I think about a third of my stash came from resale shops/charity shops, even more if you include the resellers on ebay. Some I'm sure i paid too much for but they were kits i wanted so i dont mind that. All my johan kits were from resellers and were well under ebay values.
63 plymouth - $20
rancher caddy-$25
plymouth cop wagon -$20
63 olds- $15
revell escort xr3 parts kit(missing grille and headlights) $5
frog moecedes (box ruined-glass glued)- $25
tamiya audi quattro road - $15
tamiya audi quattro rally -$20
emhar bedford olb $15
airfix 1/24 spitfire (damaged box,no instructions) $30
thats just the ones i remember, theres a load more (one entire wall of my living room is just kits 15ft by 10ft)
but if you convert the prices into £s they are even lower