@josh is referring to a portion of the model # which has SMM and SMR in it. Not to be confused with HDD
shingled magnetic recording
decoding HGST model # with those letters
MM is referring to "write optimized" - will typically have a larger DWPD or overall data write lifetime.
specs for 1616 MM variant
MR is referring to "mixed used" - or more reads than writes so lower DWPD or write lifetime.
specs for 1616 MR variant
someone somewhere put together a unified spreadsheet listing lots of details about various models. Just can't find it ATM. There's also a full break down of the various letters in the model names somewhere.
I have also observed that spec-wise and real-world-wise the MM drives tend to have much better write IOPS than the MR.
This has an impact and is observable if you have multiple storage subsystems and are vmotion'ing between them (say local DS to NAS DS). If you will only have one data-store you probably won't notice the performance impact unless you have some sort of medium-high volume transaction oriented applications or plan to write lots and lots of data in bursts.
IMO the deal you linked is meh for the MR and good if it were the MM.
In my experience the "buybest"' 's on eBay (there are a few affiliates) have a tendency to accept lower offers (a bit higher than low ball though) with quantity purchase for the items they list. I really appreciate that they usually have the drive health stats for SSD's. You might also check their other listings either if you want more drives or for a lower asking price - they tend to group drives in lots (see the first image of that listing) and probably have more drives than in that listing
(and indeed they do). If you are purchasing drives in other lots you might mention that in the message field if you OBO... YMMV.
My opinion: the 1600 series are tanks (especially MM variant). I really like them and use them as NAS served storage for my ESXI cluster.
For me as to whether it is a deal:
If ( ( (money I have to spend) >= ( (number of drives I need - want) * (cost I pay per drive) ) ) && (drive specs >= specs I need) ) then buy