If they're well structured answers that were supported by a link given to something the author is affiliated with, it's generally fine. This is what to look for:
- Do all of their answers in some way promote their product? That can be a red flag, not always, but can be. Some people just come to sites specifically to answer questions about their product. Look carefully.
- Are many of their answers that also mention their product only tangentially related to the question at hand? That can spell trouble, if someone is looking for places to plant stuff
- Do they clearly mention that it's their link? That's important, as it asserts a sort of "I hereby assert that this is given in good faith in the spirit of being helpful" thing
We positively want people whose job is coffee on this site, those are your professionals, the people that do lots of science and can answer tougher questions, especially about the brewing process. If they support some of their answers with stuff they're affiliated with, that's great. If they go a bit too overboard, moderators have means of contacting them to let them know.
With that said...
Coffee is a topic that is ripe with products (no pun intended), and we will see our fair share of folks that have come here for the wrong reasons. Fortunately, most of us have a very good ability to determine when something just doesn't seem right - and we give you the ability to flag things for moderator attention.
If you see something that gives you a bit of pause, and you seriously wonder if the person is here for the wrong reasons, flag it as 'other' and let us / the moderators-to-be have a look at it. Blatant spam obviously deserves a spam flag directly, I'm talking about the "If it weren't for the other answers in their history linking to the same site, this would seem completely benign" sorts of things.
Our job as hosts is to make people feel welcome and do our best to basically get out of their way as they participate. The fewer rules, the better. If this becomes a major problem down the road we can revisit it, but for now, I think it's sufficient if folks just keep an eye out for anything (or, to the point, anyone) going sideways with it.