I wouldn't buy a Drobo at all, new or used. A company that officially recommends to their customers that they buy Disk Warrior for when their volumes corrupt, should not be patronized. Note "when", not "if".
(personal experience... owned a Drobo for less than a year, could never trust it to be there moment to moment)
Had 2nd gen Drobo which was slow and crap but I had all drives so put them in the 5C which kept corrupting and they held me to ransom with disk warrior.. I had no choice as it was my one copy of the data.. it rescued it but it leaves a bad taste.
My experience is the same except with a 3rd Gen USB3 unit. Drobo has stopped selling that unit now. I wonder why?
I took the same drives (WD Reds, so not cheap drives) and put them in an Oyen Digital Moebius, which has worked flawlessly since.
Are we getting a bit off-track here? The OP wants to sell his Drobo not a list of stories about how ***** it is.