The easiest way to Solo merch would be Flipping. Which is basically buying an item for a low price and selling it at a high price (All done in the Grand Exchange).
EXAMPLE
I tried this on my f2p account
I brought 15k fire runes for 8 gp (Minimum GE price) for 120k. It took one night to get them all
I sold them the next day for 10 gp each (Maximum GE Price) which totaled 150k
30k Over one nice starting with 120k. Thats just a basic example tho. Also Note it takes awhile to buy and sell sometimes.
Not all items can be flipped. Usually items in high demand are or it takes way too long to sell
You can google some guides and search on youtube aswell