Excellent topic and discussion - thanks for getting this started, Rasputin!
Right now, there is no way to determine whether demand for any particular set or item comes mostly or entirely from "collectors." As bonniebeth has already pointed out, the online fan forums, such as Playmofriends, are probably just the tip of the iceberg. Many people who buy a lot of Playmobil may not even self-identify as being a collector. I don't call myself a collector even though I probably own more of the stuff than a lot of people who do consider themselves to be collectors.
What does it even mean to be a collector? Some people like to buy and display MISB sets while other folks just want to get it out of the box and display it or play with it. Others are buying it for their children (that was my own particular slippery slope
.) Some folks use Playmobil as part of a particular hobby such as wargaming, filmmaking, miniature customizing, etc. Because there are so many different kinds of "collectors" there are also many different reasons why a person might buy multiple sets or mass quantities of a particular part. All these different types of uses also make it difficult to predict which sets or parts will be most desired by all of the different types of collectors.
Different types of collectors will want different things. The MISB collectors will not want exact re-issues of classic sets Like the Victorian mansion, fence, car, and truck because that would affect the value/rarity of their own collection. On the other hand, a lot of people who want to play with their Playmobil really want those sets. A simple solution is to re-issue this type of set in bags or plain brown boxes and sell them directly from Playmobil (Direct Service.) Also, the whole "Steck, steck, steck" cry is not about wanting a set like the 7377 pirate dungeon - I already have more than enough skeletons, dogs, buckets, battlements, etc. but I do have a peculiar fondness for that grill door (only available in these "carcel" sets) and I might buy more of them individually as parts, if that were possible. I think the particular need/desire here is for unusual parts (such as the Steck balcony, the large beer barrel, clogs, milkmaid's thingie, lectern, etc.)
There is a very easy and efficient way to find out what items "collectors" will buy: just ask them. If Playmobil were to gather information about all of their customers around the world who order directly from them (whether by mail, phone, or online) and keep that information in a database then they could begin to develop profiles of the "collector" community. They could also ask those customers about their store purchases. They could ask those customers what they would buy if it were offered. As an incentive to provide this information to Playmobil, the company could offer something in return. They already provide free shipping for orders above a certain value as well as free stuff like keychains, etc. Playmobil needs a way to directly link the customer and his/her particular wants and needs to that person's actual purchases in order to fully develop the different "collector" profiles. One very efficient way to accomplish this would be to establish a Playmobil-based collector's club through which the members make all of their direct purchases and to which they could report all of their store purchases (even ebay purchases.) This type of data could be very useful to Playmobil's marketing department and the most efficient way to gather it is at the point of sale.