I claim that a specific buyer and seller can contractually agree to use a specific title company for closing a real estate transaction. Therefore, that makes it a negotiating point. Either side can reject the contract if their choice is not agreed by the other side.
With respect to eBay resellers. They probably do a lot of illegal things. But, that does NOT mean that there aren't legal ways to do what they do. Since I do not know specifically how most conduct their normal business, it has to be taken on a case-by-case basis.
The potential problems I see with eBay resellers are
1) are they selling a real estate interest on behalf of a third party? If the answer is yes, then the follow on question is are they licensed real estate agents? If they aren't, they are breaking the law in most states.
2) If they are acting on behalf of a third party, then it is illegal to direct business to a specific title company. And, it is illegal to receive a kickback as well. (Well this is true in the state of Florida, I don't know if it is true elsewhere).
So, the answer to the question stated by the OP is really determined by whether or not the eBay seller is legally or illegally acting on behalf of the seller in the transaction. Many states have loopholes that allow non-licensed parties to act as agents for owners in real estate transactions. For instance, if an estate is being liquidated. But, I think these eBay resellers use the Power of Attorney provisions of such loopholes to do what they do. It would be interesting to see it challenged in court.
But, let's say that the reseller loses the case. All they need to do is become licensed. Then, the can legally act on behalf of the seller. Then, if the seller requires a specific title company, then they are just carrying out the requirements of the seller.
I believe the laws were set up in Florida as they are to minimize unscrupulous real estate brokers from directing business to title companies and receiving kickbacks. The laws are pretty relaxed when someone wants to sell their own property. I don't think the Real Estate Commissions ever envisioned the concept of the PCC. So, they didn't create laws for protecting consumers against what they do.