Sheesh...this is a tough one to answer empirically. Truthfully, the desired mods on armor and weapons can vary greatly, depending on the template.
Generally speaking, a couple weapons can always bring in many millions:
1) Mage weapon: 1-handed weapon with -25 or lower mage weapon, spellchanneling (sc), defense chance increase (dci), and no casting penalty. For example, a twinkling scimitar with -20 mage weapon, SC, 15 dci, and no casting penalty can sell in the 50-75 mil range in Luna. Add a hit spell to the mix...you're talking big bucks.
2) Status quo dexxer weapon: 1-handed (read: pots), 40+ di, 30+ ssi, 40+ hld or hla, 40+ hit spell, and dci or hci can go for 20+ mil.
That said, every template needs something a little different.
Generally speaking re: armor, everyone needs mr and lmc, dexxers and especially archers need stamina increase, and mages (obviously) need lrc. Most dexxers wear rune beetle carapace for their chest piece, fey legs or leggings of bane for legs, and spirit of the totem or mace and shield glasses for head piece, so dexxers will be in the market for necks, arms, and gloves usually. Also, because of the arties they wear, they'll usually look for high fire and poison resists on the pieces. Most mages at lot of times wear rune beetle carapace or armor of fortune for the chest; and kasa of the ra-jin, hat of the magi, or folded steel glasses for the head piece. They'll be looking for neck pieces, gloves, arms, and legs. Again, high poison, fire, and physical resist are usually the needed mods. Seems a lot of arties have high cold/energy resist.
I wish there was an easy answer, but there really isn't. It's gonna take a few months of looking at armor (and piecing together a lot of suits) for you to orient yourself, but you'll get the hang of it eventually.
Good luck, pal. If you need any help with stuff on Atlantic, grab a UWFer and see if they can contact "Fallon." I'll be glad to look at pieces of armor for ya.