Ha! The first Fable game let you do this, buy all the diamonds from the merchant so they have low supply, the price for selling them goes up and you dump a ton back for a high price and the supply goes up and the buying price goes down again.
Flipping is great. I used to play MMORPGs and not level at all, all I would do was buy/sell and make steady percentage profits.
I remember in RS F2P going from 1 mill to Santa hat in a month or so.
Better than I expected. Though buying price!=selling price, doubt that 10% would over-compensate for it, unless the guild for some reason has the same selling price as the buying price.
Good one ! That was quite a fun original variation of the usual template of selling magic stones at the adventurer guild to get money.
These skills will probably be nerfed soon tho... ?