A lot can go into sales negotiation, pricing etc. - 60% for "used" condition is a good start, in a vacuum, but if you stick to such an ironclad rule you're ultimately limiting yourself I reckon, missing some otherwise good deals. This has potential for a good deal imo.
First of all - It's a TM NGRS, and the quality is well known. NGRSs are considered overpriced for some, but I don't think people ever really get issues with TM out of the box - some of their units have design flaws, like the M870 having a bulging gas tank or fucked 6-shot mechanism but the vast majority of TM RIFs have that "just work" quality. For these reasons I wouldn't really care if this RIF came without a warranty. I'd want a video with a full functions test, or I'd drive down to fire a mag through before pickup, but a warranty is a premium that I don't think holds value here.
The picture is shite quality but the lack of markings on the magazine (iirc those ones scratch the moment you put them in the magwell), the hop-up tag still applied to the trigger guard, and the box's condition, aaaaaand the current situation with Covid19 fucking many people's lives over, I genuinely believe this is "like new" if not "brand new".
People are right in saying that the price is a little too high considering you could get the same RIF at FS for £20 more though, though it does look like a recent price drop so I don't blame the chap for listing RRP because that's what they were going for a month or so ago.
With aaaaall this in mind... If I wanted a TM NGRS Sopmod and I saw that, I'd be happy to pay £350. £50 saving from an existing good deal, £100 saving on new anywhere else.