Backstory: My father retired years ago but he still wanted to stay active with a part time job. He spent 30 years as a cobol and sas developer and had a desire to keep his skills sharp for a few years with a part time job. He finally found one but it took several months, this got me to thinking that there may be a market for a "reverse" marketplace for retired workers.
The Idea: A marketplace for retired professionals to post their resume and skillset for part time work.
Deep Dive: This would be an open marketplace filled with users resumes, desired jobs and pay rates. Most of your users are retired are looking for a part or full time job to fill their time and make some extra money. You would be targeting those who are in it more for the experience than for the money.
Assuming that retired job seekers would be willing to wait a longer period of time to hear back from a possible employer, you would want to target that audience first when building your marketplace.
Validation: At a previous company, we hired a part time retired worker that turned out to be a wonderful hire.
Monetization: You would charge companies access to your pool of resumes. The pricing model would be recurring and could have tiers based on user/recruiter seats.