I'm going to answer this as a Canadian.
To me, value is being used as a countable noun in that sentence—which is exactly why the indefinite article makes sense.
Here's an example. Say I'm shopping and I find three things for sale, each containing multiple items. One costs $1 for two, another costs $2 for three, and the third costs $3 for four. Everything else being equal, the best value of the three is the first one, where each item is only fifty cents. Similarly, the second one, at 67 cents per item is a better value than the third at 75 cents per item.
That is the sense of value that I would use in this particular context. In other words, I consider it to be synonymous with deal.
To translate the original sentence:
What would have made this listing a better deal?
In fact, if I look at the Merriam-Webster definition of value, I see this:
3 : relative worth, utility, or importance
// a good value at the price
// the value of base stealing in baseball
// had nothing of value to say
Two of those examples make use of an article, and the first one uses the indefinite article.
Oxford Dictionaries also provides a definition of value that can have it take a countable form:
[mass noun] The regard that something is held to deserve; the importance, worth, or usefulness of something.
‘your support is of great value’
1.1 The material or monetary worth of something.
‘prints seldom rise in value’
[count noun] ‘equipment is included up to a total value of £500’
1.2 The worth of something compared to the price paid or asked for it.
‘at £12.50 the book is good value’
[count noun] ‘the wine represents a good value for $17.95’
So, treating it as a countable noun is common in UK English too.
The source specific to Australian English, the Macquarie Dictionary, is behind a paywall. However, I signed up for a 30-day trial and looked up the word value.
Unfortunately, it doesn't explicitly indicate if the word is countable or not. In looking at all of its senses and example sentences, however, the indefinite article is never used. This would strongly suggest that, as stated in the question, value is not used as a countable noun in Australia specifically.
Here is the entry for the noun value in Macquarie in its entirety:
noun 1. that property of a thing because of which it is esteemed, desirable, or useful, or the degree of this property possessed; worth, merit, or importance: the value of education.
2. material or monetary worth, as in traffic or sale: even the waste has value.
3. (plural) Mining payable quantities of mineral.
4. the worth of a thing as measured by the amount of other things for which it can be exchanged, or as estimated in terms of a medium of exchange.
5. equivalent worth or equivalent return: for value received.
6. estimated or assigned worth; valuation.
7. force, import, or significance: the value of a word or phrase.
a. the magnitude of a quantity or measurement.
b. (of a function) the number obtained when particular numbers are substituted for the variables.
9. (plural) Sociology the things of social life (ideals, customs, institutions, etc.) towards which the people of the group have an affective regard. These values may be positive, as cleanliness, freedom, education, etc., or negative, as cruelty, crime, or blasphemy.
10. Ethics any object or quality desirable as a means or as an end in itself.
11. Painting the property of a colour by which it is distinguished as light or dark.
12. Music the relative length or duration of a note.
b. the phonetic equivalent of a letter: one value of the letter 'a' is the vowel sound in 'hat', 'sang', etc.