tl;dr version - ask them for a UKARA number and the site that issued it. If they say "Huh?" or give you anything that doesn't start with the three letter code for that site shown here, followed by 5 digits, then it's bogus:
http://ukara.org.uk/#approved
Other defences apply which I'll list in full below. The realpolitick is that if you sell to someone who's a legitimate airsoft skirmisher than it's simply not going to be an issue, regardless of any evidence supplied. How to determine that is entirely up to you.
https://www.legislation.gov.uk/ukpga/2006/38/section/36
"A person is guilty of an offence if he sells a realistic imitation firearm"
https://www.legislation.gov.uk/ukpga/2006/38/section/37
"It shall be a defence for a person charged with an offence under section 36 in respect of any conduct to show that the conduct was for the purpose only of making the imitation firearm in question available for one or more of the purposes specified in subsection (2).
(2) Those purposes are—
(a) the purposes of a museum or gallery;
(b) the purposes of theatrical performances and of rehearsals for such performances;
(c)the production of films (within the meaning of Part 1 of the Copyright, Designs and Patents Act 1988 (c. 48)_see section 5B of that Act);
(d) the production of television programmes (within the meaning of the Communications Act 2003 (c. 21)_see section 405(1) of that Act);
(e) the organisation and holding of historical re-enactments organised and held by persons specified or described for the purposes of this section by regulations made by the Secretary of State;
(f) the purposes of functions that a person has in his capacity as a person in the service of Her Majesty
Also:
http://www.legislation.gov.uk/uksi/2007/2606/regulation/3/made
It shall be a defence in proceedings for an offence under section 36 of the 2006 Act or under paragraph 4 of Schedule 2 to that Act for the person charged with the offence to show that his conduct was for the purpose only of making the imitation firearm in question available for one or more of the purposes specified in paragraph (2).
(2) Those purposes are—
(a) the organisation and holding of permitted activities for which public liability insurance is held in relation to liabilities to third parties arising from or in connection with the organisation and holding of those activities;
(b) the purposes of display at a permitted event.
https://www.gov.uk/government/publications/the-violent-crime-reduction-act-2006-commencement-no-3-order-2007-firearms-measures
The regulations provide for two new defences. The first is for the organisation and holding of airsoft skirmishing. This is defined by reference to “permitted activities” and the defence applies only where third party liability insurance is held in respect of the activities.
.