If you’re trying to build back links, you really need to pay attention to the sites that want to link to you or engage in some kind of reciprocal link agreement. If the site has no connection to your subject matter, you don’t want the link – and why would they want to link to your site anyway? Likewise, some sites that link to your site may put the link on a page that isn’t linked to from anywhere; it’s just floating out there in an attempt to lure you into linking to their site. You don’t need this kind of hassle, and it won’t help you in the SERPs.
In fact, there are lots of ways that sites offering to link to you could set up the link so that it doesn’t really help you. Search engines don’t typically crawl past the third level from the home page, so if your link is put on a page that’s more than that many levels deep, it might as well not exist. The search engines will never find it.
Likewise, some sites will set up your link on a sub-domain in such a way that the page your link is on isn’t really part of the main web site. This means you’re not getting the “vote” of the main site in Google. Make sure you look at the URLs carefully.
Even if the link is out in the open where anyone can see it, there’s a right and a wrong way to do it. Some sites will just show your link without any kind of description. That’s no help at all. No one, search engine or human visitor, has any idea that anyone from the site hosting the link even looked at your site. Thus they really have no clue what your site is about.
The idea of someone looking at your site and reviewing it brings us to directories. There’s nothing wrong with submitting your site to directories; in fact, it’s still a good idea. But there are directories and then there are "directories." If a directory approaches you asking if it can link to you, it’s probably not a directory you want to be in – for the same reason that a real writer’s agent doesn’t need to go looking for authors to represent. Real directories have standards and care strongly about the quality of sites to which they link, so sites are all but competing to get in. You don’t really want to be part of a directory whose only criteria for admitting you is having a live link.
Link Trading 
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