There is a huge range of movie camera equipment and accessories for sale on ebay and an equally expansive range in the quality of the merchandise and the sellers. As a professional cameraman and keen hobbyist, I can offer the following advice:
Avoid anything sold 'as is' unless you have the specialist technical skills required to service the item. A 'slightly broken' camera may prove to be an expensive white elephant.
Ask for serial numbers when looking at professional gear and contact the manufacturer. This will help you identify stolen equipment and also perhaps even allow you to find a service history.
Enquire as to whether the equipment has been serviced or tested and ask to see a report of the results and the contact details of the servicer/tester.
Watch out for Russian cameras. Outside of Russia they are not supported well service wise and there are some dodgy traders. Also, Russian 35mm cameras are set to shoot 'positive pitch' rather than 'negative pitch' for negative film used in the west and may not be 'steady'
Don't buy: film cores, cans or spools (these are given away for free by film labs), 2nd hand film (could be in any condition - film ages like food and goes 'off'. When was the last time you had a good experiece buying 2nd hand food?).