The "120MHz CPU" bit is suspicious. What CPU is that going to be? I doubt you can get an 060 up to that clock frequency without a *lot* of work, and I think PPC CPUs started coming in at around the 160MHz mark didn't they?
Otherwise, I don't know about accelerators for A500s, but assuming they do exist with the same sort of capabilities as say A1200 accelerators, then the amount of RAM is perfectly feasible, as is the hard disk size (though if it doesn't say IDE or SCSI, smell a scam).
Anyone who had upgraded their A500 significantly would list how much chip and fast RAM it had, particularly as an obvious thing to upgrade in an A500 is the chip RAM.
Bundled items, "operating system", smelling a scam. Again with the "anyone who had upgraded their A500 that much" comment.
Though selling an Amiga with a completely blank disk is again perfectly feasible, say for example the previous owner just wanted to be quick and sure that they had wiped the machine, rather than running over the data on disk carefully.