Flipping is the art of making money through the selling and buying of items in (Old School) RuneScape. There are multiple different methods you can use to flip, from doing it low risk with high-volume items, to flipping something huge in price but only a few of them and at a large margin as they are not actively traded.
There is also another method of flipping whereby you predict items heavily used in future updates (which drives up demand), however, as no one has insider knowledge it is possible for items to crash heavily after an update after being stockpiled and dumped - something that happened with Theater of Blood (Raids 2).
Now that we have answered the question "What is Flipping?", we can go further into some of the methods. An easy method for someone getting started is high volume and low risk items. For example, feathers normally have a stable price of 3 gp, however, you can buy them at 2 gp if you are patient as other people sell off items and lower the price to sell quicker, rather than being patient and getting more money.
The problem with this method is buying limits. For any item with a buying limit, it can only be purchased to the limit every 4 hours, something that is not relevant until you get into OSRS flipping or RS3 flipping. For example, for feathers, you can only buy 13,000 feathers every 4 hours. Of course, feathers are not the only item that falls under this category (however all have buying limits essentially), but do remember you can only have a maximum of 3 (F2P) and 8 (P2P) buy/sell slots in the Grand Exchange.
To get around these limitations of buy/sell slots and the buying limit per each item, you have to make a lot of accounts and check on them a determined basis. This determined basis is dependent on how much startup GP you have on each account, as after the 4 hours, it will keep stockpiling your chosen items, you only need to actually sell when you need the GP to keep the account operating (as the basis of this is it being with low-risk items).
It can be very profitable but only if you are willing to have multiple accounts doing this each with your chosen items, such as willow logs, feathers, maple logs, water runes and more.
Typically, low volume items can be difficult to flip. This is because it can take a lot of time to actually sell and buy your items and prices can change easily from the insta-sell and insta-buy prices you have noted.
Many people opt to use sites like GE-Tracker and Platinum Tokens. GE-Tracker is a premium service and the top recommendations? Everyone who uses the site on the free trial and premium service will be flipping.
This method can be very profitable if you look at the charts generated by OSBuddy and Old School RuneScape, and also try to consider why an item is going up or down in price - if you have no idea, it's best not to buy in. It is best to use tools such as GE-Tracker and Platinum Tokens to help you in picking your own items, rather than blindly following their recommendations. Eventually, you will find your own items that aren't listed and will be able to profit highly with little work.
We hope we have answered your question "what is flipping" for either game you play, Old School RuneScape or RuneScape 3. If you have a low startup capital (<10m) look into a mixture of method 1 and 2, there are things like item ornament kits you can profit on as people sell them fast when they need GP. Once you do have a startup capital, it's up to you how you want to play things.
Flipping can be a very profitable way to sustain your accounts and alts, and easily has the potential to make more than bossing. We wish you every luck in your flipping ventures!