I have a site sell waterproof ipod and iphone case(affiliate with amazon), as the profit so low, i can't afford high ppc cost, so seo is extremely important to my site. I build link by submit to directory, swim forum etc. I am considering buying ad from text link ads. Any other suggestion that can build link fast?
Paying money for people to build for you seems to be the fastest way. Any faster and you'll be a celebrity on publicity steroids. =) Check my sig if you need help.
You can also submit articles to article directories and comment on dofollow blogs to get more backlinks.
usually low paying niche has low competition when it comes to SEO. you can analyze your competition to see how many backlinks each of the first 5 competitors have for your search term(s) and plan your link building campaign accordingly.