But if somebody on my list already bought the product, why would they buy again?
This totally depends on the product you are selling. If it's a one time product then they will certainly never buy again (only if they don't gain weight overtime, but you can't know that). But they're likely to recommend you to their friends, share your promotions, forward your emails to their friends. Include a discount code in your email message and write something like: "Hey! Thank you for buying our product, hope you're seeing progress already. (...) If you like our product, maybe your friends will too. Feel free to send this special discount code to your friends and be the one that changed their life into better etc etc etc."
Also, consider giving your already clients a bonus for sending your discount code to other people. Consider something simple, a fitness ebook, of an exclusive fitness video and so on. The idea here is to turn them into your brand's ambassadors.
And there's another way! Create a follow up product for your basic one. You sold something that helped them lose wight. That's a one time purchase. Done! But after that (after 1 month) you can send them another email. Something like: Hey! Thank you for buying our product. It gives us the confidence to go further and upgrade our product list. Maybe you want to take a look at this product that helps you stay in shape after you've lose a lot of weight. It contains healthy... things, and is recommended to be used on a regular basis. Her's a special 60% discount code for your first item etc etc etc". This way, you have a multiple time purchase product you can sell again and again, or even create a subscription box for this package.
And, regarding one of your original questions, here's something I learned overtime: Do not annoy your list. They could mark you as spam and that's bad. Send them 2-3 emails and see if they reply or take any actions. Also make sure to include a visible unsubscribe button in your emails. Let them unsubscribe. If they don't need your product, you don't need them... why waste effort on them? Instead, work only with those interested in your product. And only after about 5-6 months, you can get back to those that never opened your emails, or that unsubscribed and try again.
And the last thing: it's okay to send many emails, 2-3 per week, but keep in mind that your emails must contain some value to your list. Don't send always promotional emails. Consider to start a tips & advices newsletter; send them articles, videos, ebooks etc. Make them loyal and dependent to your content.
Hope this helps you.
Good luck!
