Yours Truly is a steady observer of eBay related LGB trains and parts activities. We are also keeping an eye on the German market in Germany and on the English/British market. We have seen quite some interesting changes lately. LGB stock is in "remission" to put it politely. Everything made before 1999 is becoming hard to find. Stock made before 1989 is almost not available anymore neither at auction nor in estate sales here in the United States. Sometimes one wonders were all the LGB stock went or is kept these days after it was sold all over Europe and the USA for more than 20 years in quite big quantities.
Now, do not turn to wrong conclusions. Locos made after 2000 in China are NOT worthless. A fine example is the Mikado which was created by LGB in 2000-2002 in Germany but then completely manufactured in China. However, some locos were assembled in Germany and stickers on a Mikado loco saying Made in Germany refer to that. All in all some 7000 Mikados were manufactured. Their value has not moved much. They were sold for approx US$ 2,000.00 and they are still worth that much. No new Mikados have been issued since then and it is highly unlikely now that Märklin/LGB will present a "new" Mikado. If getting the collector's bug make sure to take very good care of your box and the Styrofoam cast inside. It makes for 20% of the loco's value. Another good example for rising value is the yellow Capito and the MOB Ge 4/4 III which are increasingly sought after currently.
Your Famous Klaus is doing more and more appraisals and valuations these days and it is a joy to come across stock that is very well kept and dates back to 1979. Cars, long time the foster child of the LGB collector and resale market have become quite valuable if made before 1998. We have also seen some freight cars that went from being sold for US 25.00 a few years ago to an asking price of Euro 250.00 (equals roughly US$ 300.00-330.00). Here, again, it is important to have the matching box and the additional parts like coupler & spring to maintain full value.
One word from Yours Truly right out of the "Master's" workshop: please watch when giving your loco/tender to a mechanic somewhere out there. Obviously a lot of well meaning folks cannot distinguish left from right once cables are removed from their plug/stocko connector. To prevent short circuits, no sound, and other "phenomena" take a photo before you start dismantling any cables. That way the reconnecting will be a safer bet and less expensive.
And on a similar note: contrary to general opinion additional traction tires can cause harm. So can additional weight put into a loco to "improve" traction. With that you change the load on the motor which then sends an "overload signal" to the decoder and will make the loco stop. In addition it will overload the plastic gear wheels mechanically thus leading to their early demise. The motor being run in overload will heat up and will eventually heat up the worm drive which will eat the plastics gear wheel even more effectively. So, to improve traction first, last, and foremost check for well laid, even, and clean tracks. - And in case the damage is done call Klaus. He stocks German made Bühler motors and all kinds of gear wheels to keep your LGB trains running.