Hey @anonymous_user_f27400311!
As darthviper107 said, it would probably increase sales in some way, but remember that if it doesn’t have a great game design (mechanics, concept, etc), it won’t be successful, eventhough it’s moddable.
Moddable games tend to increase community participation. Games like GTA, Fallout or even Skyrim, have a strong community because those games let the playerbase to create new assets, mechanics, blah blah blah.
So yeah, moddable games expand the horizon of possibilities inside a game, and some people buy games because they have the power to change it the way they want.
Hope it helps,
Juan