general rules of thumb that I follow, as there are always exceptions:
1) non-inscrib > inscrib
2) sell perfect inscriptions only (exception being 19% forget me not) and typically only the shield inscs (10 vs, -5/20, -2/ench, +45/ench) and staff insc (apt not att, FMN pretty much all i bother with)
3) sell only perfect mods (no exceptions - that I know of anyway), I typically only bother with +30s for shield, swords, axes, and 20/ench for swords, daggs, staves, and 20/20's for swords pretty much. Most scythe mods sell well, but i don't farm NF so unless I get by way of unid buying, I don't get scythe stuff

....again, you can keep what you want if you have the tolerance for spamming wts <=3k items. Don't bother with bow mods, or 15^50's -- not worth the time selling imo. Exceptions are: demonslaying stuff for sword/axe, odd-ball slaying mods -- I'll typically keep those for a bit.
4) if an item doesn't stack and it's value is <5k, merch it after a few hours (or w/e your threshold is) of kama-spam(unless you have lots of mules or don't mind constantly updating/checking wts thread on here) -- although I tend to break this rule because I'm an idiot who doesn't follow my own rules lol
5) buy good deals if the item is stackable and can be saved for another day of selling (perfect ex is tomes -- after farming event w/e's farmers will sell of stacks of sin or mes tomes for 80g/100g ea....buy them up and sell later for 200g/300g ea, similarly with sweets/alc/etc.)
6) don't bother with "cheap" minipets, they are cheap for a reason...unless you have lots of cash (ectos) and an entreprenurial spirit for high-end mini speculation, don't get into the mini-trading game!
7) buying UNID golds can be a worthwhile investment. again, best done on event w/e's when farmers are aplenty, and you can pick them up for 7=4.5k (or lower!)...follow rules 2 and 3 when you find mods worth keeping (and 4)
8) materials -- salvage for iron, granite, fiber, and dust...these are consistently good selling materials. especially important to remember if you get into buying unid's as certain items salvage into buttloads of the above (especially with "measure for measure" inscript on em')
9) gold, dual-mod canthan skin shields -- keep these! in these waning days of GW, most players (who have been at it awhile, myself included) have accomplished and purchased most of what they want/need in preparation for GW2 and are now biding our time perusing the wtb's and kama sell channels (or w/e) looking to spend our 'disposable income'. As a result, a lot of people have found a niche item/weapon that they want to collect. Most of these collections include canthan, dual-mod shields!! Just keep what you get, and at the least get a PC on it/them before you merch - at least until you get an idea of what to keep and what is worthwhile to try selling.
10) where was I? Oh yeah, if you are going to farm - stay in Cantha, or do the SC's (dungeons or elite areas). No point farming NF if not dungeons, if you intend to sell what you find (unless you are selling UNID's, or materials of course!). Most of what is wanted that is of the highest value (besides the uber-rare end-chest stuff) is going to be found in Cantha (bo staves, dragon staves, shields, 20/20 skins, etc.)
I think 10 rules are enough for now. I'm sure others will have much to say, too, but hopefully my general rules that I use, and my little mini-guide for trading/collecting will hep you on your quest to become GW's first billionaire lol
GL and most importantly, have fun!!