According to TrustPulse, activity notifications (such as purchases by other shoppers) have been shown to increase conversions by up to 15%. These typically appear in a small popup as purchases happen, at the bottom of the screen.

There are different types of activities you can notify your users about, for example:

 

My recommendation is:

 

Below are some of the shopify apps that offer these types of notifications:

 

Below is a comment from one of the members of my Ecom Convert group, which gives another perspective:

 I can handle a cookie/GDPR popup, or even 1 opt-in popup or slider, but not constant screen interruptions. **Especially on mobile!**

There’s not a lot of screen on a phone anyway. Taking 10% of my viewport with constant popup notifications because other people bought something is annoying enough that I usually leave. I’ll never use them on my websites, for sure, lol.

 

And some thoughts by Lori Appleman over at Redline Minds:

The odd thing is that absolutely no data exists on this. The only data is produced by companies that sell it. My thoughts – this is something we only see on small, inexperienced stores. It isn’t something used by companies large enough to A/B test everything, nor is it a feature larger stores want (we work with several). It is our belief that these are not only worthless but since most shoppers know many are fake, that they hurt credibility and thus lower conversions.

That said, this is absolutely something to test. The problem is that in order to A/B test, you are going to need significant traffic – in the range of 5000 to 10000 visitors or the data is worthless. Given this tool is primarily in the small business realm, few stores have enough traffic to host a meaningful test. This would also require a tool that is easily set to only show 50% of the time, and to be able to measure the overall conversion rate for those that saw it vs those that did not.

 

If you do set up purchase notifications, here are two mistakes to avoid: