I agree with you; it's "buyers guide", meaning guide for buyers (rather than guide of buyers). This is a compound noun rather than a noun phrase. A sign of that is the main stress on the first part, "buyers guide" which is typical of compounds, but if it were a possessive construction, main stress would be on the last word: "buyers' guide".
A noun phrase has only one determiner, at the beginning. Compare (1) "I dropped my buyer's guide" with (2) "I dropped my buyers guide". (1) has a noun phrase with the possessive formed by adding apostrophe s to the noun phrase "my buyer", which has "my" as its possessive determiner. The structure is
[NP [Det [NP my buyer] 's] [N guide] ]
But the structure of (2) has the direct object NP
[NP [Det my] [N [N buyers] [N guide]] ]
You could probably use a hyphen in (2), if you wanted: "my buyers-guide" since the compound is a word.