Do the examples 'buyers guide' and 'teachers lounge' carry an apostrophe. At first glance they appear like possessives, but actually the first words are more like descriptions. My instinct is 'buyers guide' rather than 'buyer's guide' or 'buyers' guide'. This is different to a question about children's books, as that earlier question is complicated by the plural not ending in S.
Well, I could not disagree more with @Greg Lee if I tried. The guide is for use by all the people who might want to buy, namely the buyers. Therefore the guide is in that sense the property of the buyers. A clear case of genitive plural and, hence, "buyers' guide".
The whole contentious area of apostrophes with company names has nothing to do with this question: the company founded by Mr Sainsbury has a legal name, along the lines J Sainsbury and Company Limited. Everyone calls their shops "Sainsburys" (with or without the apostrophe if they are writing it down). If you look at one of their shops you will see the single name "Sainsbury".
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.