The theme has to be customizable
WordPress themes authors are great at showing beautiful demo layouts. However, It may get much less beautiful when you want to customize the said theme. Many themes look good with their own pictures and texts. Then, it may look less good when you replace the pictures and the text.
Don’t focus on free themes
Numerous times, I have noticed that free wordpress themes had major issues that need to be fixed by changing the code. For example, some free theme that was advertised as being responsive but it didn’t work on mobile phones or tablets. Sometimes, there are SEO issues with the category titles. Most of the time they don’t have theme settings panel and every customization has to be done in the CSS and PHP files.
Paid wordpress themes are not perfect but there are always better than free wordpress themes.
The least CSS and code change
The objective would be to find a theme that needs little changes. It helps to save time that you can use to create content (picture, video, text) on your website.
A theme with a live demo
When you choose a theme that is already used by a popular website, it is one of the best proofs showing that it is worth buying this theme. The theme sellers use the number of sales as a marketing argument. It is not the evidence of a good wordpress theme. It is much better to look at other websites in the same niche, check the look and feel. A live demo is the most important to check if it is worth buying a theme.