Of course you want affiliates with high commissions, but they should also have a solid reputation with high conversions and low reversal rates (you get $0 if people cancel after signing up). If they’re part of an affiliate marketplace like ShareASale or ClickBank you can see some numbers there. Companies likes Amazon/SiteGround are safe bets, otherwise do your research (or track your affiliate links so you can monitor their performance). Avoid affiliates offering huge commissions since this probably means they’re struggling to acquire/retain customers naturally. This will hurt your numbers (specifically your conversions/reversal rates).
My conversion rate went from 2.5% to 8% just by including Facebook polls where SiteGround was rated #1, along with a few Twitter screenshots and Facebook conversations. Whether it’s Amazon reviews or Facebook polls, you NEED to include outside opinions – that’s why I don’t like collecting reviews on my website – they look biased. But you can use WP Review Pro to do this, allowing people to review the product/service on your site and get those review stars.
Consider selling in lots. A lot is a collection of similar items that is sold in a group. For example, if you have a collection of books, magazines or similar pieces of jewelry, consider selling them all at once in a lot. You many not make as much money as you would have if you sold each item separately. However, the items will likely sell more quickly in a lot than they would individually.