Depends, where are you leaving from, where are you going, and where are you planning to buy?
ok, just seen that this is in the aussie regional board, so i'll presume you're from here.
It *used* to be the case that you could fly out (with the goods), fly straight back in the next day and keep your tax-back, but now they've changed everything.
I nearly got burned last year when i "visited" aus from europe where i've been living. Thought i'd do the right thing and declare it when i arrived here, knowing full well that I was leaving within a few weeks back to europe. I'd even emailed the customs department asking advice about what to do. But then she didn't believe that I had because I didn't print out the email. I ended up having to argue for half an hour that I was leaving again and showing her my ticket (which she went off and photocopied with my passport) because otherwise she wanted to confiscate my camera until I left.
(I ended up lodging an official complaint over my treatment in that incident, do they try to confiscate the cameras of every tourist that arrives?)
Anyway, next time I landed in aus I just didn't say anything, walked straight through customs and got my whole kit into the country (although I can still argue that it was personal use and a year old by that point, in case any customs officers are reading this
So yeah, i'd be reading the customs governemt website for info on that.
But if you're planning on buying overseas it's simpler, declare anything over $1k you bought overseas under a year ago and get taxed on arrival (or try sneak past and get busted and it's a nice sized fine...)