People are buying and selling links all the time for blogs. Blog roll links, content links, etc. Most of these things are on personal blogs within a given niche and are generally on thebloggerssite.com or whatever. But what about links from places like wordpress.com, livejournal, myspace, facebook, yahoo360, blogger, etc? They have to fall someplace between a forum sig and a regular blog link, but I rarely (like never) see these things for sale in BST. Are they worth selling/buying? If they are worth selling/buying how should the price differ from other blogs or blog networks?
Many of the options you mention are backlinks with "no follow" which is why they are not worthwhile as they wont register as a backlink for your site. Hope i helped.
google is coming down hard on paid links. so backlink from a site that sells links (and identified by google as paid links) is discounted or sometimes can be penalized. google is getting better over time in detecting sites that sell links. any site that have "your textlink here" or "text link $10" something along that nature with lots of unrelated links on the site will be easily identified by google as a site selling links. however, there are sites who are selling links privately. buying links on those site will be ok. if someone is selling you a link on a blog, it has to be do follow. usually links on the page and blogroll are do follow and comments are no follow. so if you buy a link, that will count as a backlink. how much the price differs is depended on the site PR and popularity. i have see PR7 links being sold for ranging between $50 - $200