Doesn't Amazon use shared inventory between first party and fulfilled-by-Amazon third party sellers, so that the first party market is potentially as much a hive of scum and piracy as the third party market. I'm quite sure that's been raised as an issue many times with regard to counterfeits.
For a good example of this, look up reviews on CR2032 coin cell batteries sometime. In a nutshell, regardless of which brand you pick or who you buy from on Amazon, it seems you're very likely to get re-packaged and possibly counterfeit goods.
Look at the do a search for amazon comingling and look at the FBA forums
In recent years I've found myself swinging back towards buying things in brick-and-mortar shops, mostly because the delivery experience can be so horrible. Maybe things are better in the US, but in the UK getting things delivered is non-trivial when you work during the day, and even if you're at home, it's very common for UK postmen to not bother even bringing your parcel with them; they just leave a "while you were out" card forcing you to arrange redelivery. Delivery woes of various kinds have been a staple of /r/britishproblems since the start.
True, but I see that as a strike against Amazon. I was really pissed off recently when I found Amazon were selling the DVD/BD of a popular recent movie as a Prime exclusive; non-Prime customers can't buy it from them at all, unless they want to roll the dice on that wretched hive of scum and piracy which is the Amazon thirdparty market.
If Prime becomes effectively unavoidable, and Amazon's sticker prices get to amortize the Prime fee as "discounts", then they no longer have much of an incentive to compete on true price. As a customer, that's not even a tiny bit compelling.