I would like to stake my opinion in your case, presuming that I have concisely understood your situation.
Even if you do not have any written agreement with the manufacturer, I do not see any violation of any provision of law if :
(i) you have bought the product validly i.e. by paying the price along with taxes as applicable.
(ii) you have not used the manufacturer's identity or his trademark, in making a resale by repackaging with your own brand.
(iii) there is no patent issue in the product which restricts you to sell without the permission of the patentee
(iv) there is no restriction or violation of any law which restricts the sale of the product, without a license issued by the Govt.
(v) if you comply with the sales tax / VAT norms as applicable for selling the product.
THE BIG LOGIC ALSO BEING that once the manufacturer sells his products for a consideration, then post its delivery, it has no much of control of its subsequent usage. The governing or restricting factors arise only by way of violation of the statutory provisions. As far as the IP aspect is concerned if you are not causing any tort to them, it is now on them to make a case against you to claim any damages.
However, it would be only wise to have a clear agreement with the manufacturer so that you can save on double taxation (sales / excise).