Half term regret sale
https://www.airsoft-hub.com/item/34815-bulldog-m4
"around 4000 BB’s I’m trying to get rid off"
Then use them as a deal sweeneter along with the battery, charger, extra mag and broken speedloader, which probably won't fetch a great deal more than a tenner at a used airsoft fair. Yes, you'll have to drop the price way, way below £150. Half that isn't unrealistic.