First of all, I hope no one really gets angry with one another on the trade board. I think it would be silly to get all riled up over something like VMK items. However, I do believe that the person selling items for credits only has the right to ask for only credits. Maybe that person only wants credits because he or she really needs them to buy new things (especially with it being October).
Plus, the worth of items vary from person to person. What is valuable to one person doesn't have as much value to another. If a person clarifies that he or she wants credits, it means that that person will get exactly what is wanted instead of debating over the worth of individual items.
If you think about it though, the same thing happens the other way around. You get upset when the person only wants credits. Other people may get upset when people want everything but credits since they don't have anything rare and can only offer credits. There's never really a happy medium; there's always going to be people on either end. The only thing that you can do is to respect the person's wishes and just wait for someone else to have the same item for trade. If you want to offer an item anyway, just politely ask if it would be acceptable in place of credits. If it is refused, there's really nothing more that you can do except respect that decision.
Many people have said time and time again (and it's something I believe as well) that a good trade is when both people are happy with the results regardless of what other people outside of the trade may think. If that's the case, then you just have to accept the fact that a particular person may want credits because that person has to be happy with his/her side of the trade too.