Link building is one of the most important SEO activities but this certainly doesn't mean that you should build links at any price – literally and figuratively. Link building can be very expensive in terms of time and money. There are many costly link building mistakes and here are some of the most common:
1. Check if backlinks have a “nofollow” attribute
2. Getting good quality links but with useless anchor texts
3. Getting an image link (when a text link with keyword is possible)
4. Not using ALT text if image link is the only possibility
5. Getting backlinks from irrelevant websites
6. Getting backlinks from sites/pages with tons of links
7. Links from pages spiders can't crawl
8. Explicitly selling links
9. Linking to sites with poor reputation
10. Linking to good sites gone bad
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