Hi Muhammad,

These kind of rules are always a bit hard to manage but in general I'd say if it feels wrong it probably is.

1. Not really. If you buy an app you should only use it for one customer if you're really fair. It is not detectable/measurable but in fact every customer that uses the app should buy the app himself. The easiest way is to let your customer(s) buy the app(s) that they need and let them send you the download link or confirmation e-mail. This way you know your customer has bought it and you're 100% following the URL's for all parties.

2 - This depends on the license of the app. Some licenses technically don't allow making huge changes without disclosing them, others allow you to do anything with the app. You can find more information about licenses at https://www.odoo.com/documentation/user/13.0/legal/licenses/licenses.html and at https://tldrlegal.com/ 

3 - Yes. If you've bought an app and it does not work or you have serious issues with it you can claim a refund for the app purchase. Once you request a refund the author has 15 days to try and solve your issues, otherwise you're paid back . For more information have a look at https://apps.odoo.com/apps/sales-conditions#:~:text=Odoo Apps Refund Policy,button on his order page.

The official Odoo appstore is a bit of a problem at the point of writing. It is full with bad quality apps that have licenses issues, incomplete info or do not even work well. If you have such an issue then don't hesitate to reach out to Odoo or start a refund claim.

Regards,
Yenthe