This answer brought to you by the word "Contemporaneous".
When you dispute a charge, it costs the merchant $10-$100 depending. As such, it is a common courtesy to warn the merchant of your intent to chargeback, to give them a chance to reverse the charge willingly for free. This shop has passed the point where you owe them that courtesy.
Of course, the shop can also lie, and say you did take merchandise, or even claim that their standard deposit agreement makes the deposit nonrefundable. Most likely Visa will side with you, and AmEx is famous for siding with customers.
In the future, there are better ways to handle this.
James Comey famously said that right after he met the US president, he sat in his car and write down careful notes from the meeting. Something felt "wrong", and he felt that he would need those notes in the future. He did this note-taking immediately, while the meeting was fresh on his mind -- trying to assemble such notes later is very difficult and widely found to be unreliable. The word for immediate action is called contemporaneous. Similarly, if you are trying to take a mileage deduction off your taxes for driving, the IRS prefers seeing contemporaneous logbook entries in a variety of inks and styles, rather than a log obviously filled out at the end of the year. Hence, the verb to Comey: to write down a verbal thing immediately afterwards.
And then we take the next step: follow it with a letter.
Dear Mr. _____ (salesman),
This letter confirms our phone call today. [date is already stated above] I gave you my credit card details via telephone and authorized a $500 charge as a deposit on the 1949 Indian, VIN ABCD1234, as shown in your photo ad, copy attached. I intend to pick up the bike when I return from vacation around (date).
If any of this is incorrect, please let me know immediately.
Have the letter capture the pertinent points of your discussion. Stuff an envelope, lick a stamp, 50 cents, keep a copy on your computer, and you've locked it in.
A few neat things happen now. First, the salesman is probably not the guy who opens the mail, so more people see it than just him, so he can't sly a deal by management. You also have impact; 99% of customers do not do this, so it establishes you as someone not to be trifled with. You have contemporaneous paper to wave around in court, instead of just his.
The last bit gives it more teeth; the judge would ask "If you disagreed with what the letter said, why didn't you object at that time?"
Then, when he (by voice) refuses your deposit return, follow with another letter referencing the first. For an extra kick, send it certified mail with the camo-green reply card. (the bright green "web code" to look up delivery confirmation isn't much use in court). Having to sign for your warning letter denies them the ability to claim they didn't get the message.
All this paper makes short work of the credit card appeal, and will be a useful counter in court to their contemporaneous notes, which could be anything.