Well... you can do it if you feel comfortable developing with OpenERP. I would create a module that:

a) Creates a many2many relationship between product.product and res.partner

b) I would modify the res.partner view so the user is able to enter the allowed products he is able to purchase

c) I would modify either the onchange_product method, or the validate method in the sale.order object in order to validate the product the client is purchasing

Hope this helps