Module Development

Hi!
I am php programmer. I want develop module for opencart to will sell. If I will make module similar to already existing or almost identical, can i sell it?
without breaking the rules marketplace.

Yes, as long as you do not copy any code or text from another extension, you can release one that has the same functionality as an existing one. Just make sure yours doesn't look like a copy of someone else's.

Johnathan thank you.
Now I am understand.

I would like to point out, however, that copying someone else's extension is probably not going to sell very well. You'd be much better off creating a unique extension that does something that doesn't already exist, and fills a niche that needs to be filled. I've seen lots of developers try to copy and sell extensions like ones that already exist, and most of them don't get many sales because they aren't providing anything new or useful. Just something to think on.