Mat
Welcome to Beoworld!
Values for older Beocenbters will vary widely depending on condition, and the fact that it needs a new stylus won;t help the valuation. My very rough estimate is that you would get somewhere in the region of £30-£70.on eBay depending on condition and luck. A lot of pics with the unit looking nice will help.
My advice would be to sell the speakers separately as anyone buying the Beocenter would probably see the speakers as a 'free bonus' and not want to pay any extra money for them. (if you have ever bought or sold a car with a tax disc on you will know what I mean!)
What model are the speakers?
Simon