I’ve written before (find it in my post history, if you’re interested) that most “influencers” are broadcasters—not influencers, meaning that they have the ability to get a message out to a lot of people, but no authority over their buying decisions.
The challenge—from a vendor perspective—is to distinguish between influencers and broadcasters. It’s borderline impossible to look at post quality, engagement rate, authenticity of comments, and draw an accurate conclusion over whether or not this person could influence people to buy your products. It’s fairly easy to spot shitty broadcasters and to know who won’t be helpful, which helps to an extent, but to really know who does have the power to influence their audience, you need to try it out.
What I’ve found that works great is to go for quantity, offer really generous commissions for a limited time, and build longer relationships based on the results.
Find as many micro influencers in the niche as you can, and negotiate a simple and short commission based deal. You'll quickly learn the approximate audience threshold where they won't do just commission. Stay below that. This threshold depends a lot on the niche and how lucrative your shop seems.
Be super generous with your commissions short-term. The point is to get as many on board as possible. Obviously use your judgement on determining the fit of the audience. There's no point in wasting anyone's time.
Analyze results and see if any deals gave good results in relation to what they posted and what their audience size is. Start working on longer-term relationships with these influencers. Be honest about the fact that you can't offer the same deal as before, but definitely make sure you can offer a fair deal.
If all went well, you just validated some influencers. Rinse and repeat to build an effective group of influencers to work with long-term. I’ve seen this work wonderfully with many businesses. Good luck!