I think it's fine to allow it but if you're going to price things based on the individual buyer, you then should be forced to accept counter offers. Make everything negotiable.

$5.00 for a gallon of milk? No, I'll give you $2.50. Back and forth until you find a mutually agreeable price.

Ah yes, the old 3 hour grocery list negotiation. This must be the lauded efficiency the capitalists are always on about.