Wait, I don't understand you say something like:
"Do you think you could send me a screenshot of the account type? I want to make sure it's BC-Enabled. Here is the login link"
or how do you get them to visit your phishing site?
If it is how i described above, I think the seller would just ignore your link and go to the site himself?
Otherwise, I like the theory behind it!
EDIT: ahh ok I read the first part of your post... that answered my Q
Still I bet they would think its shady if you gave them the link, but sill good idea!