If you’re selling an airsoft replica, then you’ll be in the same position as the retailer you originally purchased the Airsoft replica from, in that restrictions in place from the Violent Crime Reduction Act 2006 will apply to you.
If you are selling an imitation firearm – or as is colloquially known as a “two tone” then you need to ensure that the person you are selling the IF to is above the age of 18.
If it is a realistic imitation firearm that is being sold, then you need to be satisfied in the belief that the person who would be purchasing the replica is someone who has a defence/is eligible to purchase said replica, as well as the purchaser being over 18 years old. Notwithstanding any age issues, the liability would more than likely fall on your shoulders if the purchaser did something silly with what he had just bought if you weren’t of the belief that the purchase was in good faith.