I don’t see anything wrong with that.
Generally speaking, designers and developers work together and it appears that on ThemeForest there are teams of people working to deliver these themes. There isn’t really anything wrong with hiring or using a design consultant to design the theme if you are confident you can develop it to a high standard (e.g. that it is well-documented; simple to customise; compatible with current and popular plugins; and finally, that you are willing to update it frequently to provide bug fixes and provide support).
The only thing I’d be slightly concerned about is that you say you’re good at “front-end” development. Are you familiar with PHP? All customisation on WP involves a consolidated knowledge of PHP. Hope this helps.