When it comes to a good call-to-action button (what a buy now button basically is) fonts, colors and shapes come 4th, 5th and 6th after placement, affordance and size.
It's all about standing out which you don't do by just making the button look good or flashy. It's placement is everything. Put it above the fold and don't be afraid to use whitespace around the button, it's the best way to put emphasis to it.
A button has to have affordance, it has to look like a button. This has somewhat to do with shape, but more with borders and gradients. If you're not using a flat style, use gradients. If you are using flat style, correctly use borders.
Size speaks for itself. Not too small and not massively big (that's just cheesy). Just know, the bigger the button, the bigger the whitespace.
When it comes to fonts, research says sans-serif fonts are easier to read on a screen, where as serif fonts are better for printed text. But it's not a rule, just a guideline. Slab serif fonts are very popular for buttons and menus lately.
The key word when it comes to color is accent. Some people choose to give it a color which is radically different from the rest of the website's color scheme. It creates an accent to the website. But using the same color as for example the header on a predominantly white website also creates that accent. The right color is a color that stands out from the background and the rest of the colors within it's immediate surrounding (x amount of pixels around it).
The correct shape of the button depends on the design of the rest of the website. If the design permits a rounded button, go for rounded, your choice. Rounded buttons or completely square, it doesn't really matter.
All that matters is your click through rate (CTR). The rules for a perfect CTR button are not set in stone. And you don't have to create the perfect button for your website right from the get go. Test and iterate after launch. Look at your analytics and try different buttons (A/B testing).
Here are some articles that kind of say the same as I just said. The fact that they're around 2yrs old doesn't matter. The theory doesn't change over night.