Below about 110, it does affect selling/buying price.

But at about 110-115ish and higher, there is no benefit when it comes
to NPC merchants.  (Not saying there isnt' other reasons to get CHA
that high, just talking NPC merchants here)

I noted a couple of years back that as long as you hit 100 Charisma,
that seems to do about as well as any higher Charisma stat.  I can get
to about 150 or 160 with my fairly lowbie Enchanter, and the extra 50
or 60 make no difference, if I recall correctly.  Either that, or
maybe a few silver better.

Palindrome

Because the "soft cap" for merchant prices is 110 charisma (my memory may be
faulty, it's around there).  Anything over that has no effect.  You WILL
notice a difference if you compare a CHA of 60 to one of 100.

EQ uses soft caps in a whole lot of places.  A "hard cap" is when a stat just
doesn't go higher.  "Soft cap" means that the stat goes up, but has no more
(or in some cases very little more) effect.  Note that the soft cap for
different uses of a stat can be different.  For instance, CHA stops helping
merchant prices at 110, but keeps helping some types of enchanter spells much
higher than that.
--
Mark Rafn    dagon@dagon.net    <http://www.dagon.net/>  

As others have said, charisma above a certain point doesn't affect
prices.  I'm pretty sure the max needed is 104.  That assumes the
merchant is no worse than indifferent to you.  (If apprehensive, the
price will be higher; at dubious, he won't even sell to you.  Of
course, you may be able to overcome that by using Sneak and coming
up from behind him.)

I believe I've seen cases where prices were affected by CHA over 104
and/or faction better than indifferent, but they are rare.  For
instance, some merchants offer worse prices than others (the one in
the newbie Freeport yard, for instance, or the Ogre just inside the
gates).  I think two of the Iksar merchants in Field of Bone are like
that, but after I got faction up to amiable or so, and CHA 140ish,
I was able to get the same prices from them as from other merchants.

CHA does have other uses, of course.  I'm told it affects how well
Charm works, for example, though I've never played a charm class.

    -- Don.

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It is because you are already over the charisma cap that affects prices.  I
believe if you went to a merchant that is dubious to you, you would find a
difference with those charisma values, but I don't recall the exact limits
based on faction as to when charisma stops having an effect.