1) The add-on/style/product is useful and well coded. This goes without saying, but if the service provided is helpful/useful/interesting to me and I think it will help my own forum, then I will likely purchase it. Sure, price matters. I support good and quality services and think people should charge reasonable prices. Within reason, though, the issue for me is the value added to my forum. If it is useful and helpful to me, I will probably purchase.
2) The reputation of the coder/seller. (I am stating this as a consumer. Some may disagree. That is fine and those that do can make their decisions based on their own criteria.) If you have a lousy track record, can't code well, or produce as promised, then I probably won't purchase. But, I have an additional criteria that I view as part of "reputation." If you post things here (or elsewhere, like adminextra, adminaddict, or on competing product forums) that are overly dramatic, delve into personal issues of XF principals, or are generally a troll, you have lost me as a potential customer. I am only one person, but I would bet that many of your potential customers feel the same way. If you think it is makes sense to go into personal issues of KAM (or anyone else, really), it makes me question your reputation. And I will not purchase a damn thing from you. Go ahead and express yourself within allowed limits. Just understand that your posts work into my decisions about how I will spend my money. I doubt I am in alone in this view.
Why should anyone care? Normally, I would say that the above are self-evident points. However, with a lot of the current drama surrounding the future of XF and several people speculating and posting about personal situations of the developers, I have seen some developers post things that I personally found offensive. I just post this as a reminder that if you want my money, now or in the future, I am reading posts and your views factor into my decisions about how I spend money. I probably am not alone in this view. Maybe I am wrong, or you don't care. If so, then good for you. I just thought I would share my thoughts on this point.