Whenever I have sold a clack or brobot I asked $50 unless I paid more for it or traded it for a more valuable item myself. I think that is the kind of price that both the buyer and seller could agree with without anyone feeling exploited. Even though, let's be honest, $50 for a keycap is still a lot or money. In some way you could accuse me that I too could not resist the urge to make a little money on the sale, otherwise I would have sold them for retail price. I think that the fact that people thanked me for asking a " fair price" (even though it is $50 for a keycap) says a lot about what they expect from people in the community (the expectation being that their fellow forum members will try and get as much for it as possible).
Exploiting the value of keycaps and making money off fellow forum members is something that is kind of accepted here at geekhack. Not passing any judgment here, but I have seen long time members of the community here selling for over $1000 worth of these keycaps. Now, a lot of times they had of course spent a lot of money to get that collection, but in some cases I know they did not. The community makes up its own mind about what is ok and what is not. And personally, I am fine with whatever, but you can not be using double standards. Everyone is individually responsible for his or her own actions, but it is actually the people that are known within the community, and that play a larger role on the forums that set the standards for what is acceptable and what is not.
Would I have sold some novelty keycaps for $50 or more if I had not seen that this is a common and accepted thing? Honestly I think I wouldn't , even knowing what people are willing to pay, because there is something fishy about it. I have other hobbies I am more into than keyboards. Sometimes, there is a possibility of arbitrage, usually because some people get the opportunity to buy something that everyone wants to have (very much like CC's in that regard). I can tell you right now that the kind of sales we see for CC's are not accepted in those communities.