A number of the parts you're after are so cheap there isn't much point trying to get them second hand.

- suitable battery connectors, you won't go far wrong with the deans type

http://cgi.ebay.co.uk/5-Pairs-T-Plug-Co ... 4aa78d9d17

- countersunk screws in small quantities

http://shop.ebay.co.uk/?_from=R40&_trks ... ountersunk

- wheels and mounts, robochallenge blue wheels everytime

http://web.me.com/robochallenge/Robo_Ch ... nical.html

- Battery bars, ebay again (although I have just used short lengths of wire in the past)

http://toys.shop.ebay.co.uk/Toys-Games- ... 86.c0.m282

I'm sure you will be able to find a dirt cheap charger on there as well.