Maybe I'm naive but I don't understand why this is such a big issue.
If someone calls you and starts asking you about backspace/wheel fitment why can't you, before you start giving out info, politely ask the person which wheel he/she is interested in and what size. From there I would ask them if they are ready to buy. If they aren't and they say they are just shopping then I would tell them that when they are ready to buy you would be more than happy to provide them with all the necessary information needed for wheel fitment.
If they don't call back then they were never really interested in buying from you anyways.
John
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