To be honest, I tend not to bother with ebay much - I've got 3 bike shops local to me, so I usually go to one of them - that way, the bike is more of a known quantity (a good shop will want to keep a good reputation) plus I've a point of contact if I do have a problem. Yes, you'll pay more for it but I personally would rather pay more for a good bike than buy a cheapy and chuck a load of cash keeping it on the road.
I paid £3500 for my 4 year old FZ6 S2 from a shop 2 streets away from me; there was an '03 plate FZS600 in (which is what I'd looked at first) for just under £2k - I bought the younger one figuring it'd be less "tired". As it was the bike I bought had not long had new tyres / chains / sprockets, brakes had plenty meat left, and it got a new MoT / oil / filter on purchase.
Have a trawl round the bike shops in your area, see whats in your price range (if your paying cash, use that to haggle on price) and be picky (especially as its your first bike)(there's plenty decent bikes out there); a bike with minor marks is ok (newbies drop bikes, and doubtless you will at some point), a bike which has "been down the road" (crashed) is something else.
The usual advice, however, still stands: if you're not sure about your way round bikes, take a mate who is.
As for the mileage, the bikes service history and general condition / treatment is more important than the mileage itself.