Here's a general guideline:
1) Merchant all white items except for super rare items (6-22+ sickles, trible axes)
2) Merchant all blue items except for perfect or near perfect upgrades
3) Merchant all non-gold, non-15% damage modifier hammers
4) Merchant all purple items unless rare (crystalline, mursaat horn bow, fleshy horn bow, fellblade, chaos axe, etc)
5) Merchant all weapons if they're non-max or lack a damage modifier
If you ID a white item, it has a good chance of upping the base merchant price buy 20g+...but only do it on white items that you got in HIGH LEVEL AREAS, its not really worth it on the low levels.