Simply replace the unfilled areas such as "[]" and "[date]" with your information.
Hello,
I came across your [product] at [location] on [date]. Would you be interested in selling it? I'd need more details to work out a price, but if you're willing I'm sure we can come to a mutually beneficial agreement.
You can reach at [email] or [phone number].
Thank you in advance for your prompt response.
Best,
[name]