I am also doing this with some good success rates of 35%. As for the other accounts, I just recycle them and use them for other competition as you stated. If I cant seem to find anyone interested with that account, I will disable GEO location, and continue to target users interested in that niche. I would then sell the account and get rid of it.
This is a great sales method for removing the distrust in social media services since there are so many scams out there.
I find that Food, Fashion, Fitness to be the most successful. A good tip for targeting a highly likely candidate would be to look if the store is offering constant promotions and sales. This is usually a sign that they are struggling, and willing to pay to boost their sales. Just today I got a mattress company, and a personal fitness trainer. Yesterday I contacted an adult website business which ended up ordering 100 IG accounts for me to manage which are all promoting their website. They have lots of money, so thats always a good route to go if you have little morals
Quantity is very important with this method. The more accounts you are running, the faster you will get clients.
A great tip is to create 1 account under each main niche. Grow these accounts as fast and targeted as possible. These accounts will be your main niche accounts. You can use these accounts to encourage their followers to follow your new account (in the same niche for the business account you are working on). This would speed up the followers on the business account, as well as provide a wow factor to the client as its growing faster. You can also use the main niche account as a CPA, and or a shoutouts also to fully take advantage of all the profits one account can make.