A couple of things to think about, if a detector bought new from the US, and is still under warranty (assuming it is transferable), it will have to be shipped back there for any work, generally at your cost. Personally, warranties don't really bother me. A lot of second hand detectors I buy are well out of their warranty period anyway, so if something goes wrong, then it will be going back to an Australian dealer for repairs, or I will have a go at the repairs myself (I find that sort of stuff fun).
I tend to like simple and hardy detectors that can pretty much go through their warranty period and beyond with pretty much no issues. A lot technology that has gone into many of today's detectors leaves little choice other than to pay for repairs, or just to bin the detector and cut your losses if out of warranty, not something you want to do with a $1000+ detector.
Hence why I like solid and easy to repair detectors like Tesoros and some of the older Whites. There have been stories of grief from owners of detectors like the x-terra series having screen and control issues out of warranty, and the cost of fixing them simply outweighs the overall cost of the detector.
Wear parts on tesoros and some of the Whites can be generally limited to control pots, switches, coils and cabling, some of which can be easily replaced, even for the amateur. Although Tesoros have a lifetime warranty, it is not transferable.
The reason I bought second hand, and bought garrett detectors, is that I am fortunate enough to have the Gold Mining Centre here in Adelaide for any repairs, so cheap and convenient for me.
If I ever bought from the US again, it will always be a detector that I would be prepared to work on, or at least pay a few $$ to bring up to scratch. But in saying that, if the detector was advertised as in working condition, then that is what I would expect, though there are some safeguards in place should the transaction go pear shaped.
I wouldn't buy an expensive second hand detector from the US ($600+), simply too much at stake if there are any major issues, and quite frankly, too much bother to deal with after the fact. Anything cheaper than that I can live with, especially if the detector is half the price of a second hand Australian unit (Wolfau's recent GMT as an example). That gives you some leeway should ot require some repairs. Second hand detectors in Australia always tend to be way overpriced, simply due to the small local market, and the miniscule numbers we see cropping up on places like ebay, compared to 100's of detectors we see up at any one time in the US. How many times have I seen 705's and Eurekas being sold for near new prices on ebay oz!
So do your research, and always ask plenty of questions to to the seller. Often it is the info they leave out that is more important than the info they supply with the item. Things like import taxes and duties on any item over $1000 will also often make a good deal seem not so good. If you do go down that track, ensure you do your homework on hidden costs before committing.
Always confirm shipping costs with the seller before bidding, as the shipping cost on the listing may be for general international shipping, and not specific to Oz. If the seller is not forthcomimg with info, leave it and look elsewhere. Also insure your package, or insist that the seller offers it in the purchase price. Always remember, if it is too good to be true, then it usually is.