Buying an used MK3S+ printer  

RE: Buying an used MK3S+ printer

Visibly different are the holders for the y-axis (bed) rods. In the MK3S the rods are fixed with zip ties, the MK3S+ has holders with a single screw. Also, the linear bearings of the y-axis are fixed with u-bolts in the MK3S whereas the MK3S+ uses clips made out of bent sheet metal (see here). Either works. Not a big deal. Personally, I do not like the newer rod holder.

The more interesting albeit less visible changes are the superpinda, the straightened filament path within the extruder and maybe the slightly upgraded IR filament sensor.

If the more expensive parts (board, heatbed, motors, power supply, display, spring steel sheet(s)) are fine, I consider the risk acceptable. Watch out for damaged rods (scratch) due to lack of grease. But then even those and the linear bearings can be changed for little money. Prusa offers spares for most parts.

Both printers are good. The differences are mostly welcome but not that big. With a little patience and some technical skills, it'll be ok. Use the assembly manual as a guide for a thorough inspection of the printer. Start using it as it is. If desired, after getting used to it, you can swap things one by one. Prusa provides an upgrade set. I upgraded mine to MK3S+. You can/have to print the upgraded extruder parts yourself. The files are open source as everything else.

good luck

RE: Buying an used MK3S+ printer

Here you can see which part have been changed for MK3S+.
The easiest is to spot SuperPINDA sensor: SuperPINDA has the bottom in black (as far I know, older PINDA v2 had bottom in white).
Bearing clips are under heat bed and can so only be seen from below. Old version (not +) had linear bearings attached with (silver) "U" clamps. For MK3S+ wider metal sheet clamps (in black) are used.
The third difference is filament (IR) sensor. But you can't see it, because it built into extruder and hidden by cover on top of the extruder.

As you know, when buying used stuff, one never knows... Other than that, the only thing that's important is how printer will be protected inside package. In any case, you should visually inspect it on arrival and check that there's no rattling or similar.

RE: Buying an used MK3S+ printer

A printer marked "Mk3s" and sold as a Mk3s+ has most likely had some or all of the upgrades done.  

The most obvious would be the Y-axis rod holders and linear bearing mounts... if they still have nylon ties holding the rods and u-bolts holding the Y-carriage to the bearings, it would be difficult to sell as a Mk3s+.

Also, the Power supply should be black.  I think  the earlier Mk3s used the old silver power supply.  Silver ones aren't bad, just older.

The differences in the extruder are a little harder to spot and are very worthwhile.

A mk3s is a good machine and is easily upgraded to a Mk3s+... much of it is 3d printed parts which you can do yourself.

RE: Buying an used MK3S+ printer

I am an experienced purchaser of "junk" Prusa printers off Craigslist and eBay.  Be very leary.  I have purchased 8 MK3S printers off eBay and 7 ended up being the real thing.  One was a clone being sold as an original.  If it says it is an Mk3S, it might be the real thing.  If it is an Mk3S+, it is either the real thing or a clone that was upgraded.  

I still have 4 of the 8.  The rest were repaired and flipped of sale.  All of the ones that I purchased are still printing and working well.  I typically only buy from people who are well-reviewed and rated.  be learning of people who are new to Craigslist or ebay.  

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As for determining which printer you have, look at the parts: part by part.  Start with the PSU - if it is silver, you have either an upgraded Prusa printer or an upgraded clone.