I was reading something the other day (don't remember where sorry) and it said people went through three stages of search when buying:
- They're thinking of buying and want to see choices.
- They research to find what they want.
- They buy what satisfies them.
If you're looking for sites to leave/buy/barter links, knowing where your buying public goes in any of these three steps would be smart. Look for sites and blogs that host product reviews, DIY guides, testimonials etc for items in your industry and negotiate for a link. If people are searching for info on services/products chances are they'll find these sites and your link. Finding those sites should be fairly easy, do a search using your keyword + "DIY Guide" or "product review" and you should find candidate sites.