Currently willow logs sell for 8-10 gp on the grand exchange. This price does not appear to be rising anytime soon thanks to the great deal of autoers. On the general store guess how much these willow logs sell for? 16 gp for the first 50 you sell they sell for 16gp each.
Due to the way the new general store works you can only sell 50 at a time for the high price of 16 before it drops down to 8. And you must wait 30 minutes or so before being able to resell for 16.
But this method is still highly efficint for small gains of money on any character especially pures who might not have much cash.
P.S. Only do this little bits at a time. It will guarentee you 150% return on money but a considerable cost in time.