Big picture stuff: as a GM, you want things to be fair, so if a player puts more work into roleplaying but the player makes a bad decision, should that affect his PC? If someone who didn't want to roleplay just walked up and made a check, would they have succeeded? If a different player went through the extra effort of roleplaying and did well instead of poorly, would you have given him a commensurate bonus?
In other words, incentives matter. If you want your players to use a technique, whatever that technique is, you should make sure they do materially better when they use that technique.
Another thing to watch out for: hidden information your players have no way to learn about. On important checks, arbitrarily penalizing players for not knowing information they had no way of getting usually creates acrimony. On less-important checks (say, negotiating with a merchant...) you can provide mild or temporary penalties as a way of making personality traits or backstory especially tangible. If people don't feel like they're being punished for not knowing things they can't know, especially if they get a chance to make things right later, they're usually fine with whatever you as GM are doing.
These are just some things to think about as you progress in your GM abilities. Every GM has their own style and own priorities and own answers to the problems of running a game, and in time you'll figure out which answers you like the best.
This specific case: it sounds like a minor check. Nobody's going to die because you offended the merchant? You provided feedback to the player--this guy really loves his wares--and the other players understood it. The player misread that, or ignored it, and the DC went up. That's how these things usually work. The player rolled and didn't make the check. Is that all accurate?
If so, it sounds like you're 100% in the clear to me. There's not a lot else you could do to make that better.
Sounds like there's a larger problem with the player, though. Bruunwald gives very good advice here--communication is important!
If the bard's player is always this problematic, you should probably sit down with them one-on-one and talk about it. Something to the tune of "Hey, can we talk about the game? I'm human and I'm not going to be perfect. I'm going to make mistakes. But sometimes you're going to fail, and that's by design. You've been arguing with me a lot when things don't go your way, and it's been really distracting to me and the rest of the group. What's going on?" The player might have different table expectations. They might need to learn how to play or get used to not succeeding. (I see this a lot with people who get into the hobby through video games.) They might simply be a jerk. Or there could be something else going on. Not every player is going to be a match for every table and sometimes it's just best that certain players don't play for certain GMs. There's not really any shame in it--I have excused myself from games before where I knew that the GM and I were just not going to work out. Maybe that's what needs to happen in this case?
Talking to the player and figuring that out might be better than letting things keep going as they are.