Im not an expert on merching and if u have a better item to merch feel free to post.But the item i found u can easily make money with and there is little risk for deflation.
P2P- red chinchompas- If you look at the G.E chart you can see red chinchompas has a steady rise and fall so this would be great for you to merch. I would buy them 3days after its fall in price and wait another 3days for the max profit. So you can buy them for 330 and sell them for 370,that might not seem alot but as there cheap you could make 2mil if you had enough money with almost no risk.
F2p- A really good item to merch for f2p is wine of zammarok. It has a rise and fall like the chinchompas but it slowly increases in price. You should buy them 3days after its fall and wait 8 days. So u would make 280 per wine. WARNNING: Once the wine of zammarok slowly rises to about 3k it will fall sharply to about 1.9k!So do not merch when its at 3k
Hope that helps