I think the idea is to not buy stuff until at least six months after you've finished your initial training. That'd be about six months after you arrive at your first posting. Depending on your trade, that could be about six months, to a year, after you start.
There are multiple reasons for this advice. Usually, stuff bought off e-bay or anywhere else on the internet is junk that will fall apart rather quickly in real-life field conditions. For the most part, they're selling these things to airsofters, walter mitty types, and young soldiers who don't know what they need yet.
Stay away from anything labelled as if it is the type of stuff issued to "SAS", "JTF2" or anyone else like that. That's a marketing ploy trying to separate fools from their money. Sadly, it usually works.
Give yourself the time to figure out how all the issued stuff works in the real world. Only after some serious familiarity, one might justifiably consider purchasing civvy kit to "augment" the issued stuff.
My two kopecks.
Sorry, I forgot to mention this.
As for being allowed to use your own rucksack at your first posting, it depends on your unit, and based on my personal experience, that's rather unlikely.
The stuff people are getting issued now of days is pretty darn good. Don't be so hasty to replace it with something off "flea-bay".