The question is a bit vague. /u/kvothe_the_jew's answer mentions selling performances and selling recordings, but there's also the possibility of selling sheet music and lyrics. Can you clarify what you're asking about?

Autolycus, the trickster/peddler character in Shakespeare's Winter's Tale (~1610), includes ballads among his wares. This is pop culture rather than court culture, and the phrasing indicates that what he's actually selling are lyrics rather than music:

Why, this is a passing merry one and goes to the tune of 'Two maids wooing a man:' there's scarce a maid westward but she sings it; 'tis in request, I can tell you. (Act IV Scene 4)

if you consider paying for the performance "selling" then we have been paying for music since ancient times when musicians where hired to play or people would leaves bits and pieces for street performers. if you are referring to selling items that can artificial produce music then that would have started after we invented anything that could record someone playing and then play it back.