I will just point out that regardless of whether they should or not, if staff refuse to sell you a ticket, there is pretty much nothing you can do if you need to travel imminently.
Get the ticket beforehand or online if I were you.
I'm constantly surprised this is still permitted... I'd have thought by now they'd have found a way to stop it- surely just replacing "any permitted" with "not stnsd airpt" would do it?
In days of yore there were tickets routed Not Stansted, at least from Cambridge...but they were priced exactly the same as the Any Permitted, thus eventually withdrawn.
The main disadvantage of buying to/from Audley End rather than Stansted itself is that the only period return is an Off-Peak, which isn't valid to arrive into Liverpool Street before 10:00, nor to depart Liverpool Street before 09:30, nor between 16:29 and 18:34 (inclusive).