dont sell any accessories you don't see in the shops... i.e. ele charms/rings/catalogs...
pretty much don't sell anything that doesn't say at the top of the screen, "...this will sell at a premium!"
things to sell, credit chips, incentive chips, moogle puppet, cactuar doll, plush chocobo, perfume, gold dust, gold nugget & platinum ingot, all of which say at the top of the screen, "...this will sell at a premium!"
as for upgrading, its a good idea not to upgrade anything when you first unlock the upgrading option.... weapons that you can upgrade (because they are the best weapons) Taming pole for fang/rigels for sazh/lionheart for light/umbra for snow/belladonna for vanille/hawkeye for hope.
accessories that you will eventually need to upgrade anyways would be warriors fist/sorcerer's seal/diamond bangle, all of these require expensive components to upgrade to final tier... might as well start on them early...
when upgrading, (always save before upgrading/buying stuff.) use 36x Barbed tails to get to x3 multiplier... then use superconductors to level it up (use as many of these as needed to reach max level, if you use too many, reload from last save)
once you become rich, to level weapons you want to use Ultra Compact reactors instead of superconductors...
here are some good links to guides.
ff-13.info/accessory/
gamefaqs.com/console/ps3/file/928790/59358
gamesradar.com/f/final-fantasy-xiii-cieth-stone-missions-guide/a-2010031011391394090
and here are some images of weapons..


This post was edited by MrSleepin on Apr 27 2010 10:55am