I buy all over. One of the main ones is Bandcamp, particularly Bandcamp Fridays (>700). Local store rarely has anything interesting. There are few favorites when I travel, like Plattfon in Basel, 16Tons in Zürich, original vinyls in SF, and Millennium music in Portland.
The few places I have never bought anything are eBay, amazon, or general retailers (Walmart, etc.).
Re used mint, that is impossible, because mint implies unopened/unplayed. So used does not work in the context of mint. Only second hand shrink wrapped with outer cover pristine may get a mint. Alternatively, it was graded wrong, which is not uncommon.
Never bought any of the "audiophile" pressings (Bluenose, UHQR, etc.). The music is not interesting. I prefer odd ball stuff like Brannte Schnüre's "Muschelsammlung" with record noise and clicks part of the recording. I appreciate their sense of humor. Needless to say, all personal taste. I rather buy artistic packaging, like the friendship editions of 128 label or the Volcan 1/1 editions. That's something worth supporting! The 128s literally have my name engraved in the wood boxes. Much cooler than any super duper half-speed 45 rpm virgin vinyl 454 g pressings of the Blah side of the moon.