I'm currently using the official osrs GE API to get some items:

http://services.runescape.com/m=itemdb_oldschool/api/catalogue/detail.json?item=554

But I can't see any information about the amount traded in that json. The amount traded is provided on the webpage (the graph all the way down) below, but not in the json above

http://services.runescape.com/m=itemdb_oldschool/Fire+rune/viewitem?obj=554

How do third party sites like rsbuddy get the amount traded? I'd like to use the osrs GE API and not rely on other sites as they're often down. For example, I could use 

https://rsbuddy.com/exchange/summary.json

But I'm not sure if that's updated at all.  The rsbuddy link says fire runes are sold and bought at 5gp, with an average of 1.6m traded (today?) while the osrs api says the price is 6. I went in game to test, the price is indeed 6 at the GE for both buy and sell. You can see why I'd like to avoid third party providers.

 

So how do I get the amount traded? 

OS/RSbuddy's GE trade data is based on the people who use their client, that is where the "amount traded" data is from (to my knowledge). So that is why the official osrs GE wouldn't have it.

That's what I was thinking, thanks!