SEO Backlinks for Wholesalers

[Trey Crusey]

So just starting to dig into SEO since I see more promise in online marketing rather than DM. In doing some backlink searches on various competitors sites i noticed they were getting links from other wholesalers in different markets. Some had whole lists of other "proven" wholesalers in other markets and some had hidden the link in a blog or something. Sorry if this is a newbie question but is this something you just reach out to these site owners and ask if you can add a link to my site and i'll do the same? Referral program or people with connections nationwide? Any ideas or has anyone done this before? Is it common practice?