cell phone business

I do not understand your question.  It sounds like you want to be a reseller of cell phones.  As to whether or not you can buy with cash and without invoice, in terms of record-keeping that is not advisable.  If the phone is stolen, the Uniform Commercial Code article 2 allocates what is called "risk of loss"in accordance with well-developed legal principles set forth in that article.  Your best bet is to have insurance to cover theft and other casualty losses to the phones so that you can be reimbursed if anything should happen.