No you can't unless it is an advertised term of contract. I think you are misunderstanding the law. The 7 days applies under the Distance Selling Regulations and only applies to items purchased online or by mail order where you are unable to ascertain the quality, suitability or otherwise of the product. Purchase in a shop does not have this right by law as you have the opportunity to examine it before purchase.
I do believe that Argos stopped the '30 day money back guarantee' at one point, but have since reintroduced it, possibly by public demand, hence changes to some of the terms and conditions.
Perhaps that what as already been pointed out. People using the items for special occasions, then returning them for a full refund. With the item then being returned to the original supplier/manufacturer or the on-line Argos Outlet via eBay?.
Personally, I have had some recent problems with deliveries via Argos Outlet. Provided with a tracking number for Yodel, which Yodel didn't accept. Then told to try Royal Mail, which again the tracking number was refused. Parcel was then in the system for a week (and complaint) before a re-direction eventually got the item delivered. another item delivered by Yodel and was left over a gate, and in the rain as a 'SafePlace'!.
Yes Argos do have a 30 day return policy but is is a voluntary one and not statutory so they can vary the terms as they see fit so long as they are available at the time of purchase.
Yodel are an absolute joke as other threads on this and other forums will confirm. I specifically now state on any order that delivery by Yodel is not acceptable and that if the supplier does not accept my terms of purchase the order must be cancelled.