I don't own an Aura HD but I'll chime in with some thoughts mostly to agree with a few bits and pieces that people have already said. :-)
You won't need a screen protector of any kind, there's no glare and the only dust that gathers on e-readers is on the shiny plastic bezel that surrounds the screen! Definitely get a case though, the fragile glass substrate beneath the screen is the issue with ereaders and their fragility. Treat them carefully, and be aware that any flexing/twisting forces on the ereader, as well as anything leaning on the screen, are what causes them to break, not necessarily dropping them.
Unless you want to be able to carry your entire library with you all the time and will use it to read primarily manga/comics, then the SD slot isn't a must, since you can fit hundreds of standard books on them without needing additional storage.
The value of the Aura HD is set by the market and I think between $60-90 for a used Aura HD, and maybe around $100 for a refurbished model sold with warranty is the kind of price you should hope for. I think the Aura HD in particular is a model that holds its value better than most, as it still has its attractions for some (great screen and the SD card slot).
You might have to be patient to snap one up for a really bargain price. The fact is, there doesn't appear to be that many available used (not here in the UK anyway), although Canada seems like the ideal place to be able to find one, even if you think they are over-priced.
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