even eBay succumbs to the concept of buying in bulk. therein the more you buy at once the cheaper it is.
selling single minis can get combersome and cause more hassle than you may have time for, and may not sell all models.
selling units is a good idea. it allows for a compelte addition to an army with little work. and better painted thingsare still in a form of bulk so cheaper than buying singles.
there is up and downsides to all these forms.
as a buyer when i needed some armies i bought full armies or large bits collections. a $1700 space wolves/dark angel army for $400. how could i pass that up! since it came with the Skullz promotion standard bearer, and 12\' statue Brother Osric. heck for the vehicles alone (3 land raiders, 2 predator, forgeworld vindicator kit, forge world vehicle doors, whirlwind, venerable dereadnought, and 3 land speeders) that was retail nearly $400!
also with the Tau its a crazy market. Crisis suits may be able to be sold as singles as they are a must. Fire Warrios/krot units should be sold as units.
you may want to sell any devilfish with a unit of fire warriors that includes a transport or may choose to sell them individually. hammerheads will probably sell good as singles, as well will broadsides.
special characters should probably sell for singles.
other types may require finessing. steal units, drones, pathfinders; will probably needs to be sold as units, or maybe all together.
the new skimmer i don\'t really know about, but i think units would be good for it as well.
bits should be sold as a single lot, unless you have a hammerhead that can be converted between railgun, ion cannon, skyray modes. then you could include those bits with it for modification. but of course that should garner a high cost since its now a kit with upgrades.
did i leave anything out? i don\'t have the newest codex so i may have, but i will give my opinion on any leftover pieces if i know what they are.
this of course i how i would look for them if not in an entire army lot.