I'm sorry to say, but you really won't get much for an A500 on eBay. With all the accessories, maybe £10. The problem is that there are millions of them out there, and they regularly sell on eBay UK for a £1 - £2 + postage. Even
this auction for an A500 + another 3 A500's for spares + games and PSU's + A590 HD + A520, Scart cables etc only sold for £10.50.
If you want to get the most money, split everything up - even to the point of taking apart the A500 and selling off the socketed chips individually.
If you sold everything in one lot, you'd be lucky to get £10. Sell it off in parts, and sell the games individually at 99p starting bid you might get £50