You have a few options;
First, since you have title to the bikes, and what you describe, you are clearly still the legal owner (since the contract
for sale was never completed).
First, if you can retrieve the bikes yourself without causing a "disturbance" then go for it. This is called "self help"...example, say the bikes are parked out front of the home. If you can take them without incident, since you are the owner, no one has any legal claim to get them back. That said, the law requires this happen without disturbance...so you can not use force to accomplish this...that would violate the law.
Next, you can report this to the police and request they assist. They may. Or they may not. Most likely they will tell you that it is a 'civil matter' and to use the courts. But you can sure try. I would.
Next, you can sue. You are the owner...so you can sue the person who has the bikes now AND the person you sold them to, both in small claims
court. You can only collect the total value of the bikes...but you can get a judgment against both and seek to enforce the judgment on whichever person has income to garnish or money in the bank to take. You can sue them in small claims court so you would not need to hire a lawyer.
Let me know if you have more questions.