Trading

To get better items or better prices, trading with other players will be almost unavoidable.

About quantities
It is possible to get good deals on certain quantities of items depending on location and the buyer/seller, such as 12 coal for 1,000 coins.

Experienced players trade most items in packs of 1,000, it saves them tons of trading hassle. Reliable and hassle-free 1,000 traders quickly become popular and so can get somewhat better prices.

Also, when you buy in large quantaties, the game forums hosts many sellers. Though all players can access the forums, only members are allowed to post.

About powerful players
A level 90+ smith may buy coal for higher than market price if you sell in chunks of 10,000. Very few players actually have the patience to accumulate that much, and the smith might be impressed. On the other hand they may have several mining clans already mining for them and ignore you as "one-time pocket change".

Smithing is the most coins per hour you can make as a free player, but it takes extremely long to actually turn a better-than-miner profit. Many f2p players who reach 85+ mining stick to simply mining runite as leveling their smithing to 90+ can cost more than the the player is willing to spend(not only money, but time). Casual players should stick to something less demanding like mining iron.

How to check by yourself if an item can be traded
Method 1: Try to withdraw it from the bank in noted form. If it can't be withdrawn as notes, you can't trade it. This method does not work with stackable items. For stackable items use method 2.

Method 2: Check for the price of the item at a general store. If it says you can't sell this, you can't trade it with other people.