I did some flipping to get started on my cash stack, but as Mojo said, most people won't tell other people what they are flipping, mostly because it creates more competition.
I am no expert on flipping, but here are some tips:
- Don't think too much on the margin at the start, you don't want to get too greedy at the start
- Fast traded items are very safe, but margin is not the greatest. Basic elemental runes, for instance, is a good option. You can see the volume of the trade of an item in the grand exchange side, e.g for fire rune: http://services.runescape.com/m=itemdb_oldschool/Fire_rune/viewitem?obj=554
- Margin changes relatively quick for items that trades at medium/slow speed. You may find items with very good margin at one time, but may be completely different couple hours later, so be careful!
In the end, all I can say is that you need to try many items out and see which one is good for you. Try items that are frequently used on skills, or pvp or even PvM.
Good luck!