I'm going to wade into this with an official Finius endorsed opinion, because I do that sort of thing.
I don't see this as a case of "avoiding attention", in fact, that's not a good thing to seem to want to do at all. The Royal Mail, if they open a parcel, are already curious, if you try to avoid suspicion purposely, they'll probably see that as something in itself (as in, who are you and what are you hiding).
My advice would be to do this: Do not label the exterior of the parcel with anything other than the address it's going to and a return address. You don't need to, you're not required to, nor is it anybodies business what is in there. Inside the parcel, simply print out an invoice, fold it neatly and package it in the same "compartment" as the IF.
On the invoice, you simply need to be put what you'd expect to see on an invoice (postal addresses, contact info etc, price paid and what exactly was sold - so if it's an imitation firearm, say that's what it is). If they open it up, they'll read the invoice and will know what they're dealing with. If you try and hide behind "sports equipment", they'll want to know why you weren't 100% squeaky clean and honest with them. This is how I'd do it:
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17/9/11 - Mr Blah, 178 lolguns road, airsoftshire, le3 th4x,
Return contact information - Mr sellsthings, 69 sellguns road, secondhandshire, om4g gnu, 01234 567890
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£178 - Imitation Firearm (model: TM AUG Civ)
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Violent Crimes Reduction Act compliance checked? - YES
Check completed: Buyer Age has been confirmed via photo ID
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I'd literally leave it at that.