Thank you all very much for your response.
neweden and moneyer12, thank you for your offer of help. Since this for now just an idea, we don't know how the structure will be, but if we will decide on some sort of Moderator position with certain responsibilities, we will be happy to have your help on that.
hbofill12 - the rating system should be thought out, of course, for a new platform. May be, as suggested here, some minimum entrance barrier of swapping ratings in order to be able to sell. In any case I think that the management of sales will have to have more clear, rigid and enforced rules on Colnect than in swapping, so the assurance of a fair trade will not rely heavily on the ratings system alone.
kljaja - I understand your concern for the character of the site. This idea for a market place comes to improve the conditions for collectors on Colnect, and make the process of selling and buying, which we gather is quite common as it is, more comfortable. The sense of community and the service of it's needs, I think, is the best and most important thing that Colnect built over the span of it's existence, so it will not be disregarded.
The reason that I put this idea on the forum for discussion is in order to see how we can make this in a way that serves the collectors community. I believe we can do it in such a manner.