"Sold" in the meaning of "bought"?

Solution 1:

I believe that "Sold" in the first and second comment is short for "I'm sold on (something)", meaning "I'm convinced of (something's) value."

This seems to be an American-specific phrase; a quick look at Google NGrams for the phrase "I am sold on" shows it appearing in American English around 1880 and peaking in popularity in the 1940s. The same search for British English... shows nothing at all.

The earliest example I see - from 1917 - gives the sense of the phrase quite well:

"To say that I am sold on the Essex only half expresses how I feel after an eight hundred mile trip through western Tennessee," writes WH Claypool, sales manager of the Memphis Motor Car Co.

Solution 2:

Sold and bought do indeed mean the opposite thing. In the case of the blog posts, the posters are probably using 'sold' as an interjection, to mean "you have successfully sold that to me". In other words, it means "you have convinced me of something's worth, and so I am prepared to buy it".