Now buying sponsored tweets can definitely be an amazing thing to do in order to get the word out about your service or product launch. But there are things you have to focus on in order to avoid sellers who paid for all their followers just to sell sponsored tweets.
Some things you need to focus on in order to avoid pointless services that are selling sponsored tweets.
Look to see if the profile is verified. A lot of bigger profiles will take the extra step to verify their profile so everyone knows it's genuine and actually them. There are some users out there, like I said above, who will just build a million fake followers in order to sell sponsored tweets. People building fake followers will most likely not take the extra step to get verified.
Make sure the person tweeting out your message is in the same niche, or markets to people in your niche. This will increase your conversions ten fold and make you a happier person after the tweet goes out.
Check the engagement of the tweets. If no one is commenting, favoriting, or retweeting the sellers previous tweets then it may mean they are tweeting a ton each day and people can't target in on one specific tweet to comment on.
If someone is tweeting out a few hundred times per day, avoid them even if they have a real profile with millions of followers. Your conversions will be garbage because people are less likely to see your message since it will fall off of the sellers profile fairly quick. Typically the cheaper services will be hit harder and less likely to get you a sale because more people are buying it. Target the tweets that are $15 or less because not as many people will be purchasing these. Some people have success with the $10 tweets, but I don't usually risk it.