eBay is the well-known Internet marketplace for virtually everything. Many secondhand sellers operate on the site, while some artists elect to sell their music there firsthand.
Example of an eBay link: http://www.ebay.com.au/itm/371416174953
The unedited version of this URL was http://www.ebay.com.au/itm/Affection-love-songs-compilation-various-artists-1990-CD/371416174953?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20151029142714%26meid%3D2defa889e9b54766a317c52c7aca5ae0%26pid%3D100005%26rk%3D1%26rkt%3D6%26sd%3D360918473580&rt=nc.

Discourse discussion: https://community.metabrainz.org/t/ebay-listings-as-purchase-for-mail-order-links/209266