it gets me nowhere- but i often just mumble to myself pointing out how much it's going to be to get it running. usually more than the asking price, and i use 400 as what it's worth running and good shape, because around here that's about true. alot of times people inform me of 'how much they're worth, because of their rarity" of a maxi or something then get pissed off when i have a picture of 20 of em, and mention not one was over 200$.
another thing that really gets me nowhere is saying that i never pay more than 200$ for a non running ped, which is true (except for the clunker monti sold me. purchased sight unseen, lesson learned!). many scoff at the thought of me not paying 400 or so for their non running moped, and i leave. i'm sure my bullish stance gets the person after me the ped alot cheaper. walking away is fun sometimes though.
the final thing i do that really never helps is offering to fix it if they're only selling it because no one will fix it. i like to think that they might still want to keep it if it works- this leads to all kinds of problems whether they have you fix it or not, because they now have your phone number, and TONS OF BROKEN SHIT THAT'S NOT MOPEDS.