$($a$)$ In the scenario described above, the first step in forming a contract at the trade show between Robert and Cassandra is the offer made by Robert when he showed Cassandra the range of remote$-$control toy cars. This offer was made when Robert assured Cassandra that the toy cars would be popular with kids and make a perfect gift. The next step is the acceptance by Cassandra when she agreed to purchase $500$ toy cars at $$30$ each. This acceptance was clear and unambiguous, demonstrating her intention to be bound by the terms offered by Robert. The consideration in this contract is the promise by Cassandra to pay $$30$ for each toy car, in exchange for which she would receive the $500$ toy cars from Robert within $3$ weeks.

The element of intention to create legal relations is also present in this scenario, as both parties appeared to have intended for their agreement to be legally enforceable. This can be seen in the written sales order signed by Cassandra, which outlined the terms and conditions of the agreement. Therefore, all the essential elements of a contract, including offer, acceptance, consideration, and intention to create legal relations, were present in this scenario, indicating that a binding contract was formed at the trade show between Robert and Cassandra.

$($b$)$ Under contract law, Cassandra may have grounds to avoid performing her part of the agreement and refusing to accept the toy cars and pay for them. Firstly, the delay in delivery caused by the strike at the manufacturer's toy factory may be considered a breach of contract by Robert. The agreement was for the toy cars to be delivered within $3$ weeks of the order being signed, and the delay resulting in the delivery after the Christmas selling season constitutes a failure to fulfill the terms of the contract on Robert's part.

Secondly, the fact that only $300$ toy cars were delivered instead of the agreed upon $500$ may also be considered a breach of contract. This shortfall in the quantity of goods delivered could entitle Cassandra to reject the partial delivery and refuse to pay for them as per the terms of the agreement.

Furthermore, the discovery that the toy cars are of a kind banned by Product Safety Australia due to a high lead content poses a serious risk to children and is a significant issue that may provide Cassandra with further grounds to avoid performing her part of the agreement. Robert's knowledge of this ban and failure to disclose this information to Cassandra could be seen as a misrepresentation or non$-$disclosure of material facts, which could void the contract or provide Cassandra with the right to rescind it$.$

In conclusion, Cassandra may have valid grounds to refuse to accept the toy cars and pay for them under contract law due to breaches of contract by Robert, including delayed delivery, shortfall in quantity, and the presence of banned toy cars with a high lead content. These issues may entitle Cassandra to seek remedies such as rescission of the contract or damages for any losses incurred.