Kind of a well-known one but, avoid the greedy merchants.
Merchants like the female inside the freeport bank, boomba the big, or the male just inside the qeynos gates will give you 50% less for your wares and charge you 50% more for theirs. This is especially important when every silver piece counts early on. Don't sell your precious cracked staff to these guys (see list below)
https://wiki.project1999.com/Greedy_Merchant