I've come across this several times but can't find the explanation. Maybe someone on this forum knows.

E.g., the sentence for translation was:

I did not buy the cheap fountain pen.

My translation uses "o":

安い万年筆を買いませんでした。

Yasui mannenhitu o kaimasen desita.

The book's answer uses "wa" after fountain pen:

安い万年筆は買いませんでした。

Yasui mannenhitu wa kaimasen desita.

So why would one use "wa" instead of "o" if fountain pen is the direct object/accusative case? Is it because fountain pen is the topic as well as the direct object so "wa" trumps "o"?

If I put "watashi wa" at the beginning would I then have to use "o" after fountain pen since "wa" has already been used?

Thanks.

The sense that I get (and I could be wrong) is that "o" merely states that you didn't buy a cheap fountain pen, whereas "wa" gives me the impression that you bought a fountain pen, but it wasn't cheap.... maybe because you like quality fountain pens

The quick answer is that "wa" is frequently used with negative verbs. Here as squarezebra says it sounds like it's implying that you bought a fountain pen but just not a cheap one.

The presence of "watashi wa" would make no difference; two "wa"s in a sentence is common. The "wa" particle basically has two functions (which are not necessarily exclusive) -- marking a topic, and showing contrast. The "contrast" meaning is what makes it used with negatives, although in some cases the use of "wa" with negatives is almost idiomatic and has no strong contrastive meaning.

You should remember that labels like "direct object" and "accusative case" are grammatical categories from Western languages (primarily Latin) and will not always apply to Japanese. People use them as shortcuts because grammar allergy is endemic, but you should always be prepared for Japanese grammar not to match with the Western categories.

(EDIT: One additional note that is not reflected in most textbooks is that in speech, it is very common to use no particle at all in marking subjects and objects, so just to say something like 安いペン、買いませんでした。 This is not considered sufficiently formal in writing, but this so-called "zero particle" has been around for at least 1300 years and is really an indispensable part of the language.)

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Arupan Wrote:安い万年筆を買いませんでした is grammatically correct, but it sounds strange. If there's は instead of を it doesn't really give the impression that the author necessarily bought a more expensive pen but:
①that he/she just didn't buy it
②he/she bought something else instead (could have been a regular pen, for instance)

If you want to use を and not resort to tactics like omitting particles/words and so on (as @yudantaiteki pointed out many people do use this tactic and it's fairly common when you speak with friends, but it's kind of cheap, especially if you don't really know how to construct a whole normal sentence), then you should write something like this:
1) あの安い万年筆を（結局）買いませんでした.

Or even better:
2) あの安い万年筆を（結局）買わないことにしました。

Some small variations:
3) あの安い万年筆を（結局）買わないことに決めました。
4) あの安い万年筆を（結局）買わないと決めました。
5) etc...

Other options:
6) あの安い万年筆を（結局）買うのを止めました。
7) あの安い万年筆の購入について気が変わりました。
8) etc...

Thanks, this is really helpful.