I'm a fan of second-hand gear. I think it's a great way to increase one's purchasing power and play in the bigger leagues. I've bought and sold a fair number of second-hand subs and, apart from a KEF with dried-out capacitors which was a simple and inexpensive fix, touch wood they're all going strong TTBOMK.
Would I buy the Wharfedale used for a £60 saving? Maybe; if I knew the seller well; and if I knew they weren't a bit of a muppet.
IMO the risk goes up the cheaper the product. Someone running their first sub -
and, with respect, an entry-level product such as the Wharfedale - may have run it hard to get
fat bass from a box that really wasn't designed to energise the whole street. A lot then is down to the person you're buying from: Do they know what they're doing with the gear, and have they been running the thing flat out? Of course, they could be sensible and left the sub set as the AV receiver wizard had it adjusted.
You mentioned about a demo. That's a good idea. Have a look around the sub first, and pay attention to the position of the control dials. Volume at 50-70% = Good. Volume maxed out = Not so good. Where the sub is being run from an AV receiver, you could play the wide-eyed innocent and ask if it's difficult to set up and say you've read about "something called the speaker set-up menus" but how that sounds confusing. Your aim here is to get a look at the menu screen with the dB levels. What you're trying to see is if the sub level is running much higher than the rest of the speakers. It's difficult to generalise because room conditions play a part, but if you're seeing the sub's dB level in the +6dB ~ +12dB range then it could suggest that either the receiver is trying to compensate for a sub that isn't big enough to drive the room, or the owner has tweaked it up for effect.
I know this all sounds a bit doom and gloom, but I look at it from the money side of things.
You might be saving a little over 1/3rd of the cost new, but in today's world how far does £60 stretch? For me, it's a couple of meals out, or half a tank of fuel, or a fairly modest night out with the lads. If this was a £1000 sub, or even a £500 sub, then a similar proportion saving starts to become a more significant sum: It's £375 off a £1000 product, and my gut feeling is that a £1000 sub wouldn't need to be run hard in the first place, and that the sort of buyer for a £1000 sub is probably going to be a more conscientious owner. I don't know if the same can be said of the average owner of a £160 sub.
There are some vids on Youtube of the sound of subs bottoming out. If you haven't come across the phrase before, then bottoming out describes what happens when the bass cone reaches the end of its travel and kind of 'hits the buffers' so-to-speak. It's not just a problem for small subs. Here's a big 18" Velodyne being over-driven by the owner.
https://youtu.be/nsGnGDUsAKQ?t=60 Eeek!