You have to remember that although a bristlenose should sell for £5 in an LFS (£25 in P@H Lincoln. LMAO) that a lot of fish do not sell quickly and quite often you can go to a fish shop 1 month after your last visit and see the same fish as your previous visit.
Therefore when they offer you 50p/£1.00 they are weighing up the selling price against the length of time they will have to feed them and provide homes for them. Also it's not just a case of your fish being healthy and bred locally!! Will the LFS be able to keep them alive for 1 month.
They don't have too much of a problem with Neons, Danios, Livebearers because they are selling them for £1-£2 each or less and they are sold before they have to fed too much and the risk of some dieing is much less too. A bristlenose isn't gonna catch the eye of a beginner when they can get 10 cheap fish or 1 bristlenose for the same price. The beginners or just bad fishkeepers are the ones that buy the most fish because they have more fish die on them, whereas someone who has researched, learned from their mistakes and generally keeps a nice clean balanced tank doesn't buy too many because they are 'full up' due to nothing dying.
I would sell them in the buy/sell/swap section on here or if you can sort out the postage part and heatpacks, styro boxes etc, sell them on ebay.
I'm always seeing 'where can I buy a BN' because places like P@H (apart from this very rare £25 one in my local store) don't have bristlies, infact they very rarely have anything except commons, albino commons, and gibbys.
andy