I would say unless you have a very big budget and are willing to invest in it, then I would stick to finding good services and building relationships with existing sellers. Theres the costs of buying the accounts, software to manage the accounts, proxy costs. Then if accounts are deleted from taking actions to quick or in a suspicious way, or even if you just get caught lacking by a new social media update = money lost on investment in accounts + buying new ones.
Yes you could start off with a small number of accounts, but you have to consider if you will have enough business to cover the monthly/ongoing costs of maintaining the service.
As a reseller you only have to test out the services and switch service if there is an issue with delivery or quality. Just my two cents.