In reply to the original post...
1. We always said we wouldn't charge for our own weapons. We're not. We're using this as a way to reward community members for work done.
2. It is no longer a "mod". It is DLC taken in as part of the game itself. That is one of the many little headaches we've got to work on to find ways to make this all work sweetly. Random individuals trying to charge for odd bits of code/art are still in breech of the EULA.
3. The weapons don't change gameplay - they are "side-grades". They all have stats similar to existing (FREE) weapons.
4. There have been a couple of comments about "pay to win". Um - this is a CO-OP game, not competitive. You're not playing AGAINST someone who can get a drop on you because they've spent money. No-one can "beat" you at all - with or without any extra weapons. The weapons aren't any "better" than FREE weapons anyway. That is just knee-jerk reaction.
So - the weapons are effectively cosmetic anyway. If we've missed anything and one of them is OP, we'll reel it back in - just as we would with FREE weapons. Have done, in fact. We're doing it as an experiment, related to the Steam workshop experiment, seeing if we can give people decent ways to reward community members who do a boat-load of work - that meets the quality bar as well. We picked this one as the experiment as it is reasonably simple to handle. And Paypal buttons on random websites are largely useless.
We have looked at other ways of doing it and will continue to do so. Ultimately, we want the players to be able to choose what is worth it to you - and what it is worth. And then we will always have to inspect for quality and sanity. But that is still a little way off.
So this is an experiment: some weapons that do NOT make anyone "over-powered", but are a fun alternative to the (50+) (FREE) stock weapons. No-one NEEDS to buy them. No-one will get any more "powerful" with them. The IJC guys will get some reward for the time they've put in and we will learn things from the experiment, adjust plans and move on. What direction that takes depends in part on this experiment, on part what we can afford to do, in part on what the player community is interested in.
But there is a limit to the amount of free content we can generate (we already do 3 events a year, new maps, weapons for free, some DLC characters to help pay for it) - so we really do want to encourage the broader community to make more content that fits, that is of high enough quality and is attractive to the players. If we can find ways for people to earn a bit of cash off providing that content, then the player base will wind up with even more content to play with. That is one theory, anyway