I tend to agree. I think a *500POP plate was something that was really common with the early adopters but isn't so popular with people buying them now.
RobinPJ makes a good point, the whole plate thing makes it all a little ambiguous. If I liked the look of the car (I do actually) and wanted to buy it I'd probably be put off by the fact that people selling personalised reg's always tend to want zillions of pounds for them. I would perhaps put a price on it or as Robin says, chuck it in as a freebie or if you genuinely feel that it's worth something, just include it in the price or sell it separately.
Slightly off topic, does anyone remember the plonker on ebay who we all messaged who was selling a 500 and wanted something silly like 15 or 20k for an 08 500 back in late 2009 purely because it had the plate K155AHA and she was thinking that some A-ha fan would pay thousands for a plate with the name of some 80's band on it.......