
That always makes me smile! Playmobil do not really understand the importance of adult collectors, although there was an all-too-brief Playmobil Collectors Club which they axed, to the anger and confusion of collectors. Lego has embraced the adult collector market, even marketing sets as for age 18+ (because of 'difficulty,' not because of 'adult' themes!

), but Playmobil hasn't really understood the potential. Judging by membership of forums like this one, there are many PM collectors in Germany, Spain, France, Belgium, The Netherlands and increasingly the UK, US and Canada. And Portugal has at least two!
If you enter a search on eBay, then scroll down on the left hand side you can select 'sold items' and this will show you what people paid, though not accepted 'best offers'.
What you pay will depend on how obsessed you are, like Oliver said. But there are many sellers asking unrealistic prices. If it's a recent set, it's worth searching online retailers to see if there is a better deal.
If it's a spare part, it's worth checking if it's available from DS for a lower price. Some sellers seem to buy spare parts and resell for inflated prices on eBay, though Playmobil do not allow this. Check on playmobil.co.uk (scroll to the bottom of the home page and click on 'find spare parts') to see what is currently available and prices. As US DS is notoriously poor, you could ask a Playmofriend (

) to make an order in the UK and send over to you!
If you don't mind paying the shipping charges or shopping in German, eBay.de has probably the best range of Playmobil, though many sellers there don't ship internationally or use PayPal.