And plz dont answer 'When someone is willing to give something nice for it'

I've heard that sentence once too often :)

It really depends on what item, and how many sockets.
2/3/4 socket armors are nice for runewords, IAS jewels, and Perfect Topaz's
for the poor man's MF gear.
My favorite armors are usually Boneweaves, Scarab Husk, etc.  The heavier
ones are in less demand, but if you can find the right Pally or Concentrate
barb on the lookout for the high defense stuff, sometimes you can get a good
deal for it.

I'd say a war axe isn't even worth selling.  I've seen people looking for:
Berserker Axes, Glorious Axes, Decapitator Axes...I think one more but can't
remember what it was offhand.

Long Swords aren't valuable either, people like the Phase Blades, Cryptic
swords, Colossus blades, Colossus swords, and a few others.  Mostly the
elites though.
The value is going to differ though, depending on what runeword they're
wanting to make, of course.

+2 skills on an Ammy has potential, depending on theother  mods.  Get 25
resist all and some leech, and you could have yourself some trading
ammunition.

+3's are harder to trade (at least for me) because they are usually blue, so
less tasty secondary mods.

short

Only two kinds of socketed items (or white items) are worthy.  One is for
low level duelers, another is for highend runewords.  In both cases,
ethereal superior ones may score higher than a regular one.

Let's take boneweave as an example.  People may want one for their CoH or
Enigma runewords.  Since they are putting in extremely expensive runewords,
most would look for the best possible armors they can get.  They'll usually
want high def low str ones.  In this respect Boneweave is slightly inferior
to an Archon plate; both have similar defense rating, but Archon needs much
less str to wear.  Also, you should look at the def on your Boneweave, which
has a range from 399-505.  If it's at ~500, you may get a chance; if it's
around ~400, better sell it to Charsi (and buy Bing a drink); if it's
superior, esp >+12% def one, it's even better.  Ethereal ones with def bonus
(600+ def) may score high for those thinking of making highend runewords for
their mercs.

For low level duelers, they usually want max socketed items with least req.
Long sword is not a good choice for them because of the dex req (they use
Negal/Angelic rings for the AR and don't nearly use dex at all).
Furhtermore, if it is something that can be easily bought from town folks
and get socketed by Larzuk, then it's not worth much.  Some of these LLD's
are very generous but recently as I've seen, most generous traders have
disappeared after selling their stuff on paypal.  Good luck finding one.

For rare amulet, the guideline is very simple.  If you find one, moan, and
would save game right away so you won't lose it to lag, or so excited that
you will think about building a char for it, then likely it will have a high
trading value.  If you pick up one, and think that you can get something
decent from a stupid arse, then, there aren't that many rich stupid arses
out there.

Actually, it's not just rare amulets that can be valuable.  I have a +3
necro summon +92 life magical amulet that I got a lot of enquiries (from MH
users peeking at what I was wearing), and they offered big for it.  But
then, since I play HC, the +life part of the ammy means a great deal to
other HC players.  In SC, it would likely have a much lower value.

Bing

I'm not sure but I know a 6-socket ethereal CB or berserker axe are much
coveted. I'm still looking for either.

A +2 ammy with 2 resists is good. Great if there's MF. A +3 to a skill tree
magic ammy with 20+ MF is my favorite.

It all depends.  Here's an example.  Say you find a socketed staff with some
good skill bonuses.  If it has 2 sockets, it could be valuable because you
could make Leaf out of it.  If it has 4 sockets, it could be worth it for
Memory.  If it has 3 sockets, it's pretty much worthless, since there's no
good runeword for staves that has 3 runes.  Other things that are worthless:
anything with just 1 socket.  There's no runeword that has just 1 rune.  No
sockets is better than 1 socket, since you can go get Larzuk to add sockets,
and if it's a white item, you'll usually get more than 1.  Sorc orbs with
sockets - useless since the only runeword that could possibly work is Venom,
and that's pretty much pointless in an orb.  Plus, almost none of the
runes/gems/jewels you can get will do much good for a sorc in a weapon (with
the almost solitary exception of Rainbow Facets.)  Anything both socketed
and ethereal - with the exception of 5-socket melee weapons (for Eternity)
or 6-socket weapons (for BotD).  It's possible somebody might want them for
a merc, but that limits the trade value significantly.  (Thus, any weapons a
merc can't use that are socketed and ethereal are pretty much completely
out.)  Also, items which are normal quality (as opposed to Exceptional and
Elite) probably won't be worth much, unless they're
staves/wands/scepters/class-specific items with good spell bonuses.  Even
Exceptional ones won't be that popular.  Thus, 6-socket long sword - crap.
Especially since you can make one yourself with a cheap cube formula.
Either use it yourself (might be fun filled with Emeralds) or chuck it.  I
don't think anybody will want that enough to give you something for it.

Darrel Hoffman - Lvl 26+2 Game Guru
(Imagining BotD on a Long Sword...)