The "easiest" way I can think of is to have enough of an alternative source of incoming USD into your PayPal account (eg via your own website) to  at least minimise the additional amount required to cover buyer purchases through eBay (and I put easiest in quotations as I know that can be easier said than done, especially if you are in Australia and your primary market is the US). 

You could look at various Visa / Mastercard debit rates, and if you can find one with lower costs than allowing PayPal to do the conversion, you could save that way as well (i.e. by funding your payments with that card and using them to perform the conversion instead of PayPal), though it will still likely be a bit more expensive than just having a flow of USD into your PayPal. 

I think there are actually some bank accounts that allow you to hold a USD balance, so it might work out cheaper to convert AUD to USD that way, and then use a card attached to the USD balance to fund the purchase - I really don't know in this case, what's available and what's possible, just kinda throwing an extra avenue out there that might be worth exploring - maybe talk to a few people at various banks / financial institutions and see what they say.