yes, I bought some rolls of 1st and 2nd class stamps years ago; I'm still working my way through them and they must have cost half what they do now. But I don't know about 'investment'. There's not really a secondary market in them (unless the shopkeeper actually sells them); better to see them as saving you money on postage later... assuming postage continues in its present form.
You have to get the ones with 1st and 2nd on them rather than ones with a price on them, the latter continue to be worth the figure on them only. (I vaguely recall some proposal to change the rules on this but I don't think it happened.)
shopkeepers of varying sizes sell many things, aog, not necessarily stamps; on the other hand, they also mail things. So it was unclear whether you meant buying for resale or for posting things. My answer covered both possibilities.