The tip is simple - buy near-perfect weapons for greatly discounted prices then customize them without worrying about the fact you can't sell them, because you didn't spend much on it in the first place.
EXAMPLE: you want a fiery Zodiac sword of defense

Fiery Zodiac Sword of defense (+5), with 15^50 - maybe 50-100k

Fiery Zodiac Sword of Defense (+5), with no inherent mod - about 5k.
Customize to +20% dmg always, 10g

In this case you would save a lot of money and do 5% more damage. (sometimes 20% more, if health is down)