figure the prices out yourself, I have a simple guide on doing so on my youtube channel, User: Elektrey
Also I Find merching big bones, dragon bones, frost bones, prayer pots, dragon longs, rune scim, rune 2h, d scim, or any kind of high demand item good.
Also calculate the profit before buying/selling it might not be worth your time.
f2p try runes, lobs, maple logs, rune armor.
Just keep in mind the amount limits and price boundaries.
If anyone has any trouble PM Me
Also just a pic of my success, I've just started merching: