A certified reconditioned Kickr Core is $780 on Wahoo's website right now. They don't come with a cassette so factor in another $50 or so if you don't have a spare laying around. This is probably the safest route of buying a used Kickr.

These things are pretty rugged, so I do not think there is much to truly worry about. You mainly have to trust the seller's description that it is lightly used and in perfect working order. Also, shipping is probably expensive, so weigh that into your total cost. As noted, a refurbished 2018 KICR directly from Wahoo is $1,000 USD.

My n=1 says I'd never buy a used Kickr that didn't come with a return policy.

We have two in my house. I'm on my third return. My wife is on her second.

My experience is it works great right up until it doesn't and then it's just a very heavy and expensive paperweight.

Wahoo support was great but it does suck having to pack, ship, and repeat the setup as a result of the returns.

You also will not be covered by warranty if you need it. I just had to buy a new power supply for a KICKR climb that I bought new-in-box but from a bike shop routed through eBAY. A second hand purchase is almost certain to void any warranty.

The 2018 kickrs are particularly problematic according to wahoo. I'd steer clear of buying this particular version of the kickr, especially used 2018 (V4) models. ...I've heard good things about the V5 model (the one with the axis feet).

I bought my 2018 Kickr off FB Marketplace last year during the pandemic when you couldn’t find them in stock anywhere. The seller stated that it had little use (I checked his Strava and verified it was only used about 20x for ~ 300 miles). I think I paid about 800 and got a Wahoo mat with it. I haven’t had a single issue with it and I’m on level 26 now on Zwift so I’ve used it quite a bit. If you are comfortable with the seller, I’d go for it.

Smart trainers adhere to what is known as a bathtub failure curve. They are most likely to fail very early on in their life cycle or following heavy abuse. Wahoo kickrs are no exception and there is no way of looking at a unit and knowing if its going to be one of the really solid unites. If the trainer was really only used a few times it is still in the danger zone of failing pretty quickly. Ideally you would want a unit that has seen moderate use. As others have noted if a unit is a good one it will last and will have a lower risk of failure that a new or nearly new unit.

Overall the warrantee is an important part of the cost on new smart trainers. Unlike with bike frames the earlier failure rate is expected to be significant so companies have to add extra margins to account for the cost of replacements in the original price. Without the warrantee the consumer is taking on a lot more risk and I would only buy if the price reflects that. Once a trainer is properly worn in however the imminent failure risk drops as does the value of warrantee.