I've noticed this across the board. A lot of stuff is still shipped by Amazon, but lots of third-party sellers. Of course, Amazon does intermingle inventory of their own with that of third-party sellers, depending on what's in your local warehouse, but I still prefer to buy stuff that is shipped AND sold by Amazon, which is getting harder and harder.
I rarely buy car parts on Amazon, but I buy A LOT of computer equipment... and while Amazon has been my primary source of such items for like a decade now, I've found myself using Best Buy and Newegg a lot more recently. Of course, both those sites have "marketplace" stuff on their website too, but I always filter by what's shipped and sold by them directly as well. I have the Best Buy credit card and the rewards are decent... so even if Best Buy is $5 more, it works out.
