We mainly buy things we want to keep and inevitably, as others have said, when buying mixed secondhand lots this means we acquire a lot of things we don't want or need. Some things are not very popular with anyone in our family (for example Special Agents, Construction, Airports), so these are easy to part with.
My wife and I became more 'commercial' in the last couple of years, actively buying secondhand PM for resale. This meant targeted buying of certain themes (zoo, hospital, school, castles, fantasy, fire, police, etc.) which seem to sell well on ebay. We have now come to the conclusion that in order to do this well, it takes a lot of time and effort and the return on outlay may not be all that great, especially as I always buy loads of missing parts from DS to make what we sell as complete as possible. If I could just bung the stuff out on ebay without worrying too much about the quality, condition or completeness, it would take far less time and make more money, but I won't work like that, so my bad! There are plenty of people selling Playmobil online and no doubt it is possible to make a living at it, but we have decided to restrict ourselves to collecting for the love of it from now on!