How old is it? Can you verify that with receipts etc?
The 280L is from the Nepton line.... which comes with a 5 year warranty.
Typically, AIO liquid coolers are warranted for their expected lifespan - 5 years is about what you can reasonably expect from any decent AIO. By their very nature, they're not going to last indefinitely.
The Nepton's launched in ~2013 or so (IIRC). If bought new at the time, you're right at the 5 year mark and I wouldn't spend any money on it.
I would suggest, if it was bought within the last 12-24 months (and you can get proof of this), then it's probably a decent cooler for $40.
If it's any older, I would avoid.
Remember, the MSRP at launch was $139, shortly thereafter was reduced to $120. So you've got a seller looking to recoup about 30% either way.
Chances are, if it were still relatively 'new', they'd be asking more. They likely know how old it is, that it's shelflife is limited at best in 2018... and are trying to recoup some money from an unsuspecting buyer.
Unless it's <2 years old, you're likely going to be spending $40 on a cooler that won't make it a year.
A solid air cooler (like an H7 or something), without the inherent issues of an AIO shouldn't set you back much more than $40 anyway.