I'm going to travel to Australia in April and I'm traveling the east coast (from Melbourne to Cairns) with a campervan. I'm very keen on surfing and have some experience. I was wondering if it's better to buy and sell a board or rent a board multiple times. And when buying, where do I need to look for a good deal?
My partner I travelled 'around the block' from March to September and rented boards in various places along the east coast. Prices vary a little, but count on paying at least $20 AUD for a day (or sometimes just a half day). To put that into perspective with buying a board: in the last backpackers we stayed at (in Sydney, Bondi Beach), second-hand boards were offered up for $45.
For cheap boards, you're probably best off at backpackers, in particular in cities where travellers commonly depart the country from. You can inspect the item on the spot and make a decision based on what you see (and try to negotiate the price down of course ;)).
Another good option would be to check out Gumtree though - it's an Aussie subsidiary of Ebay without sign up/listing/selling fees and therefore very popular for buying/selling virtually any type of product or service (note: as a side effect of that, there can be quite a bit of junk and small commercial activity to sift through first, depending on the product category). Anyways, as an example, here's a list of boards up for sale in the Melbourne region (130 ads at the moment, cheapest board costs $100).
In the end, it's just a matter of simple math and how much you're willing to spend on a board of your own.
PS: I've made the assumption that by "good deal", you weren't referring to a brand-new board. Unless you plan on living on the beach for several months in a row, I doubt a new board would be a cost-effective solution for a temporary stay.