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You would be able to take them to court based on two theories.
First, the issue of your pay would be entirely controlled by your contract. If you are a contractor, then you are paid in accordance with a contract. Failure to pay you for the hours that you worked in accordance with your contract is a breach of that contract, which gives you a simple breach of contract
claim for those hours.
Second, on the issue of buying things, if you were of the understanding that you would be repaid, you can sue them for unjust enrichment or an implied contractual basis and seek reimbursement
in that way.
Filing a police report isn't really going to be effective, because this is not considered criminal theft. You voluntarily worked and voluntarily bought these things....you just weren't paid for them, which is a civil wrong not a criminal wrong.
If you can manage it, you should file in small claims court, which only requires that you state that facts of your case rather than trying to fit those facts into specific legal claims. The court would figure it out for you. You can sue for up to $5000 in small claims. If the amount that you're talking about here is $5000, or even higher but within a reasonable amount to $5000 (like $6000 or $7000) it will be financially worth your time to file in small claims, as it is cheaper, faster and you don't have to pay an attorney.