Buying unidentified items is a slow way of making money. You have to find someone selling them, then ID them (which you have to buy the ID kits), then salvage the mods people will buy (which you have to buy the salvage kits), then sell the mods. Outside of mods like +30 health, +20% enchants, and runes like Superior Vigor, Death, Shadow Arts, etc. they won't get you more than 1k each, if that. But since you have to find people to buy them, it doesn't happen fast.
For the second part, you hint at what is called Power Trading. You buy expensive items for less than their normal value, and then sell them at the normal value, or higher if lucky. This can be done with anything, including items for collectors that have values as low as 8g each, but usually it is done with HIGH end items that cost 100K+ecto each. To do this, you have to know the current market value of the items, as well as the demand. More importantly, you have to have the money to purchase the items so you can resell them.