On buying items I typically play it a little fast and loose, I know what stats are desirable and thus expensive. For items below lvl 50 I never offer more than a chaos, and typically only an alchemy orb, if I see a piece of gear that will make my leveling experience go faster I will pay a little closer to a chaos but the short of it is I wont be using it for more than a couple hours.
For end game stuff, its a sellers market right now, unfortunately. If you see a really nice piece of gear there are 20 other dudes that also want it, so I just ask them what they want and if they ask me to offer and shoot them a 3/4 offer of what I think its worth - they will either tell you no or take you up, its not worth my time to barter with people, but some people enjoy that aspect of trading. For selling, I know what price I want and if I don't get it I don't sell it.
Some tips:
-Quality gems are typically worth 1 GCP for each 5% quality on them, this is not always the case desirable gems are much more expensive whereas undesirable gems are never worth it (auras are about as garbage as quality gems come).
-If you are looking for a 4Link white item never pay more than a couple jewelers for one, when you hit merciless you will be drowning in them.
-5Link weapons are pricey if they are desirable types (1.20+ speed mauls and thicket bows mostly) but other than that are worth maybe a gcp if you are lucky.
-5Link armors on the other hand are always pricey. Most expensive being armor or evasion, then armor/evasion hybrid and tied for terrible last place are energy shield, and armor/es and evasion/es. The armor, evasion and armor/evasion typically start at an exalted (6x GCP), the others are generally just a little below that not by much though.
-Desirable mods on all armor:
[*]+Life 50-100
[*]Elemental Resists 20-45%
[*]Chaos resistance 10-30%
[*]+Armor/Evasion/Max Energy shield (flat amount)
[*]+%Armor/Evasion/Max Energy shield
[*]Increased Attack Speed (only on Gloves)
[*]Increased move speed 20-35% (only on boots, most important stat on boots, severely reduces value without it)\
[*]Increased Weapon elemental damage (can be found on weapons, quivers, belts, rings and amulets - very desirable, very expensive 20-30%)
-Desirable mods on all weapons:
[*]Increased attack speed 15-30%
[*]Additional elemental damage (fire 30-60/lightning 20-120/cold 30-60 flat amount)
[*]Weapon Elemental Damage increase (20-30%, again very expensive)
[*]Additional Physical damage (flat amount)
[*]Increased physical damage (% of maximum)
[*]Chaos Resistance (10-30%)
[*]+Level of *weapon or elemental* type gems (Wands with +2 to lightning gems are very pricey, bows with +2 to bow gems, also very pricey.)
-Rares can have 3 prefixes and 3 suffixes (prefixes and suffixes are the item mods), Life and damage mods are all prefixes, resistances are all suffixes. An item rolling 3 desirable prefixes or suffixes is rare - and thus valuable.
http://www.pathofexile.com/item-data