Can I read e-books w/o a Kindle or a Nook?

I wouldn't buy something that poorly described unless I was desperate - in many such cases, the sellers don't have much of an idea of what an ebook is, and are simply selling a pdf file. In that case, you could read it on your computer (with Acrobat Reader) but it would lack many of the features of a real ebook, such as the ability to change font size/type or reflow the text.

There are desktop versions of both the Kindle and Nook software that will read files intended for those devices. However, these are by no means the only ebook formats, and do you really want to take a chance buying from someone who doesn't even know what they're doing well enough to provide the sort of info that any buyer would need? There are various ways to deal with most formats, but it shouldn't be your job to figure out what you've been sold.

By the way, I'd also look closely at something sold with that poor a description to make sure it was a) not pirated and b) not public domain material available for free elsewhere. The latter is commonly seen on Ebay. Another common source is self-published, self-marketed material by people without much understanding of what goes into making an ebook, in which case goodness knows what you'll be getting.

You need the program that will read the format of whatever you are reading. A lot of e-books are in PDF, so you need a PDF reader. Nook and Kindle books need the program (or app) that can read that format, but you can download those apps for you Mac for free.