I've been on 500px for a few weeks now; it does, indeed, seem to be the new hotness in terms of photo-sharing sites, but it's probably too early to judge its long-term staying power. At this point, the site is positioned as high-end content (think 1X), but with the openness of Flickr and some nice social networking features. One of the big open questions for 500px is whether they can sustain the current apparent quality level (with respect to photos submitted) as they grow in size. In other words, how do they achieve the scale of Flickr without diluting quality?
At this time, it looks like 500px is working on enabling photo sales via Fotomoto. Photo sales, according to that link, are live and working right now, but the link to activate them isn't present in the normal UI yet, as testing is still in progress. You can sell photos from either the free account or a paid ("awesome") account. Given this, and with the additional features you get when you upgrade to "awesome", 500px has a lot of potential.
I recently deactivated a Smugmug account. I found Smugmug to be great for photo sharing, but most of the stuff I wanted to take advantage of there was only available in their paid account. I opted to build my own site rather than upgrade, but Smugmug is used successfully by lots of photographers to do exactly what you're looking for. I was a little disappointed in their social networking features; they've got groups for sharing photos among Smugmug users, but I never saw a lot of activity driven out of those groups. It's too early to call whether my experience is going to be different for 500px, so I'll withhold judgement on that, I guess.
You can enable photo sales from a basic ($40 /yr) account, but be sure to check out the feature list, because many selling features (such as setting your own prices) are available only in the pro ($150 /yr) account.
Right now, the safe call is probably a Smugmug Pro account. They're a proven commodity, and they're stable. If you're feeling the least bit daring, though, 500px has a lot going for it, and you can try nearly all of it for free. Had I known about 500px, I probably would have skipped setting up my own site and just thrown all of my eggs in that basket. As it is, I'm going to stay active there for a while and see how well the site matures.