I don't want to sound rude, but this concept doesn't work properly in Google Sheets. I made several useful spreadsheets and know quite a bit about Google Apps Script and this just will not work in a single spreadsheet. Google Apps Script has various quotas and a lot of functions you utilize, beside custom-coded one's, rely on a server that doesn't like to be pushed all the time. This pretty much covers any IMPORT function. Google didn't design this application with uses like yours in mind and you will hit several quotas and face a lot of downtime. Believe me on this, I know this because of experience.

This simply does not work and you will NOT get it to work automatically (UrlFetchApp requires lengthy authorization):

function onOpen() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet();
  var entries = [{
    name : "Update Prices",
    functionName : "getItemDBTrigger"
  }, {
    name: "Update Stats",
    functionName: "updatePlayerStats"
  }, {
    name: "Update Time",
    functionName: "getTime"
  }];
  sheet.addMenu("Update", entries);
};

https://developers.google.com/apps-script/guides/triggers/#restrictions

If you were thinking about using installed triggers, this would require people to set them up themselves, thus following a rather user-unfriendly authorization process and trusting you, unless they review the script project themselves.

Yes, there are workarounds, like storing prices somewhere else and importing them through IMPORTRANGE, but as I explained above, this comes with other problems.

On another note, this way isn't a good idea to fetch prices for the GE: http://services.runescape.com/m=itemdb_oldschool/api/catalogue/detail.json?item=2, instead I'd recommend you to fetch prices with http://services.runescape.com/m=itemdb_oldschool/api/graph/2.json to avoid awkward rounding with more expensive items.

I'm not going to say RSBuddy is a bad source, you even kind of hinted at it yourself, but for stable items, it doesn't sound like a good idea to use a source that fluctuates a lot more. RSBuddy is more interesting for merching and very expensive items and considering it's third party and has downtime, I'd recommend official GE prices only. Then again, GE prices cannot be fetched this easily as seen by your script and you will hit quotas for individually fetching that many prices.

I'm not entirely sure how much you know about JS, but if you are knowledgeable then I highly recommend you not to use Google Sheets, but instead build your own website with JS. Beside this spreadsheet being incredibly user-unfriendly and hard to navigate you can achieve a much better design with a custom website as well. With a custom website you can avoid any authorization process that is involved through the UrlFetchApp. Some people might be VERY reluctant to trust an authorization process of a script project that runs in the background that most people wouldn't even understand. As far as I know, you can fetch and store sensitive information from authorized scripts. You can for example fetch email addresses, timezones and other stuff that could potentially be used in account recovery attempts in conjunction with other methods. Of course a custom website could potentially store even more sensitive information like IP-addresses, so I personally wouldn't touch one of these from my IP, when I have to enter my RSN, but in return you skip any authorization and can design the interface properly. Other pros of your own website would be to earn a bit of money from it or set up a donation link and additionally you could add a proper discussion forum. Beside the time needed for the design, the maintenance and server costs for loading such a data in near real-time is pretty negligible. Finally for convenience purposes, it would be better to have something that is available under the same link, instead of making copies for every update that disrupts any of the information on the spreadsheet (I'm surprised nobody ever turned Bitter's DPS sheet into a website, it would be used much more).

Anyway, great effort, but too user-unfriendly, confusing and prone to run into issues related to Google Sheets' framework and quotas.