basically the essentials have been said. I´d like to add little details as to how could be tested and what is worth the effort to do so:
proph/fac:
id all purples, all shields, foci, wands, q8 max items
test all martial weps out of town (while chestrunning) for -1HP reg, -1Ene reg, caster - if negative: sell, if positive: decide (for the -regen ones hit a foe to check if it´s zeal/vamp) sell as unid unless it´s easy to sell or high value (keep in mind -regen could also be 14% ones which would reduce value)
decide with staves - usually I id all q9 and all of rare skin
sell all other golds as unids
nf/eotn:
sell everything gold as unid (or if you care check for shield 45HPwE, 30HP, Vamp Scythe and HSR Foci)
id and merch any purples
just keep those that´d be worth somthing like q9 magmas etc - you get the point
presearing (my advice - I just don´t care enough to differentiate and rather just id everything):
way more difficult, but yes - there are mods worth 10a which justifies the effort maybe
id all foci
check all wands/staves for Ene mods and for HSR, HCT and +1attr mods (by using proper proff skills 10 times - by the skin you can tell proffession and for each proffession there´s just one valueable attr - so test this only) - HSR and +1 id, hct depending on attr
martial weps with high DMG go to lvl0s worms attack and note the number of the crit - you can calculate back the added percentage to the given base dmg and since there is no slaying mod that triggers on worms you can be sure it´s the inherent. if you are not satisfied - sell as unid (on an auction)
any other martial wep do the same on a charr mob - if there´s additional dmg check the worms - if there´s none: id, since you have a charr mod. if there is - check if different or same percentage (roughly - 10%tolerance) if different - again a charrmod, if not can sell unid
shields get yourself a charr axe mob and wait for executioners strike - if reduced dmg id, if not sell
note: difficult since you have to know the mechanics of how dmg is calculated, but once you got the hang of it, it´s straightforward - even though I doubt it´d be worth the effort, just saying it can be done ;D
And I highly doubt said person is really doing this, but some presortement that keeps items with higher chances for higher values is sure possible (e.g. keep all highDMG ones and just sell low DMG, or keep high HSR staves and sell low ones, keep all foci...) I mean that´s the obvious approach 
