Looking for the best moving coil cart that is around $5K used

If you are looking for somewhere to improve it is your turntable and/or tonearm. Just because your Pioneer sounds as good as your VPI doesn't mean that either of them are letting you get the best from your cartridge.

Try using a great tonearm like Graham, Basis, EMT, etc and you will see what I mean.

Good luck and have fun 

I've been lucky buying most of my used cartridges, but yes it does involve some luck. I've bought a couple of used Koetsu stones that sounded just like a new one (now that I've had a new one to compare, and Koetsu rebuilds too) and have lasted a long time. I'm long done buying used cartridges, but a bit of a gamble helped get me into Koetsu at I time I would't have otherwise ($3K vs. $9K new).

You just need to "luck" out on a seller who cleans their stylus frequently, plays clean vinyl, runs proper alignment & VTF, avoids traumatic stylus incidents, reports hours honestly, doesn't hide issues like a skewed cantilever, low-rider, channel imbalance, etc :P The high-rollers that run a rotation of multiple high-end cartridges are good candidates. And unfortunately the audiogon market isn't what it once was for carts on offer (I rarely see Koetsu stones that look any good for a reasonable price).

But like others said -- at this point look at your table/arm first, and don't forget phono stage either!

The ability of the arm to track the cartridge correctly is far more important than what cartridge! So the answer to your question will be the cartridge that works best in your arm.
 
The thing that would worry me about an inexpensive arm is the bearings. If there is any slop in them the arm simply will not be able to do its job. Quite often the reason there is slop is that the bearings are damaged, if they use points. If they don't (using ball bearings), they might have a lot of sticktion... For this an other reasons there is a good argument for going with something other than the stock arm. I don't remember all the Pioneer tables, although I've working on many, but what I do recall is that none of them were offered without an arm.

So that might be the tricky bit...

Maybe I missed it, but did the Ortofon Winfeld Ti get mentioned?  That is the one I would spend my $5K on without a second thought.  In fact it is the one I will spend my $5K on when my current Per Winfeld is up for renewal. I love the sound of this cartridge.  In fact I prefer it over any other I have heard including the vaunted Atlas.

There is no "best moving coil cartridge" and the higher price on the used market does not mean that 5k is better than 1k cartridge. The used marked is the platform where everyone could find a great cartridge for very good price. Not every auction ending at the high price. Some of the brands are overlooked, some are overpriced. And some brand (or the models) from the 70s/80s are unknown to the most of the buyers. I’ve never had an issues with used cartridges, but i’ve had an issues with new cartridges. Started from the very expensive MC carts (up to 4,5k) i slowly realized that the price is not so important when you’re looking for the cartridge that sounds the best for yourself in your own system. Practically those expensive MC carts are terrible, my broken new ZYX Premium 4D was a good lesson. Finding the good preamp for LOMC is a pain in the ass (just an extra expenses).

Your Pioneer turntable with its tonearm is perfect for MM cartridges, under 1k you can find extremely good (and rare) Moving Magnet carts. Recently i sold my spare AT-ML170, it’s hard to find any better cartridge, the price was under $800, easily compete with 4k MC cartridges! Another one has been shipped last month to Florida, it was NOS (never used) Victor X-1 with box and docs, the price was $1490 for a brand new cart from the late 70s. That Victor is also hard to beat by any overpriced MCs. They are all from the same era as your Pioneer. Personally i use Pioneer own top of the line MM cartridge (Pioneer PC-1000mkII) and i love it! I have MC cartridges as well, but i’m sure that for your Pioneer you’d better buy MM cartridge of the highest caliber! It will kill the LOMC!

Forget about Koetsu and other low compliance MC cartridges, it’s a waste of money, you don’t even have the right heavy tonearm to use them.

P.S. I really like the design of the Pioneer PL-70 mkII turntable along with their exclussive p-3. Sadly new turntables of today does not looks so good, something wrong with the designers.