Why do some webmasters avoid links, though they may be useful? It seems unprofessional.
Some approaches.
It is unprofessional. This is particularly the case when laziness results in lumping all outbound links on a single page. The better option is to insert the outbound link where it's most useful to the user. If a link only appears with a bunch of other links, with no useful context, it's not helpful.
Examine the outbound links on sites like Office Depot (e.g. bCentral and bizjournals.com). Yes, they are partnerships. However, they aim to serve the users of the Office Depot website. Everyone wins. Likewise, Amazon also links out to partners such as Target, where there's a good fit.
Commerce and linking.
Surfers are there to buy, at a commerce website. If you're building a long-term relationship, you're not threatened by their exits. This is especially true if the exits are already covered with compelling reasons to return. As mentioned earlier, if price is the issue, they'll do comparisons. Or, maybe they'll return because your site is easier to use, more helpful, has a better return policy, etc.
It remains true that you do NOT want outbound links at the crucial purchase page. But, that's quite different.
Problems with outbound links.
Do not offer to link back where it isn't a mutual match. Example? An educational software company might be a good match for a particular school's students (or a particular program). However, it's not in the interests of the education software's users to see a link to the school. The users' needs should determine who you approach for links, and who you agree to host a link.
Conclusion.
More often than not, there are better reasons to link than not. It is unfortunate to see people afraid to link to excellent resources. The reason being some perceived penalty by the search engines.
In the end, it seems if you build for the user and not for the search engine, the sales will follow. You need to make sure you are offering a product in demand, which is part of offering the user value. This has been obeserved with many commerce sites. Of course, this assumes you are following the suggestions covered here.