If you rightfully own a registered U.S. trademark, you can sell it for a profit on websites like Ebay.com.
You should not apply for a trademark with the sole intention of selling it, as you'll either lose money or fail in obtaining a trademark (or both!). But, if you do have a trademark and are interested in selling it, there are lots of alternatives to using a Trademark Broker. You can sell a trademark registration on Etsy, Reddit, Facebook Marketplace, and Craigslist. Essentially, any place where you could post a used product for sale, you can also post your registration.
Tips for selling your trademark online
Work with a lawyer.
In the online listing, list everything for sale.
Do you have any open online accounts in the name of the brand?
Do you have any business entities in the name of the brand?
Do you have any leftover inventory?
Do you have a URL or other digital items related to the brand?
In the online listing, post all trademark information including registration number.
Describe your preferred transfer process.
Are you going to mail the paper registration?
Are you going to submit the assignment record yourself? Or does the buyer need to draft and submit the record?
You are likely going to have to exchange emails if your online selling platform doesn't allow attachments, so possibly create a dummy email.
Tips for buying a trademark online
Work with a lawyer.
Make sure the seller is ceasing all activity.
Just because you are purchasing the trademark registration doesn't mean the other party is out of business. You need to make sure they aren't hanging onto the URL, inventory, or other items with the branding. If they are, this can hurt the validity of your new brand name.
Double-check the seller's story and registration number.
Ask for detailed sale records and invoices.
Just because someone says they own a trademark registration doesn't mean they actually do. While that may seem obvious, it can be hard if someone has an LLC or business entity with multiple members, where the seller may only be a co-owner.
Insist on a complicated transfer process.
While the paper registration certificate doesn't hold value, you might want it mailed to you to make sure the seller isn't engaging in a Mona Lisa Scam (where they sell multiple people the same trademark).
To transfer the trademark you need a contract detailing all of the information and an assignment agreement to record with the US Patent and Trademark Office. Thus, you need the other party to sign the record.
You are likely going to have to exchange emails if your online selling platform doesn't allow attachments, so possibly create a dummy email.
Every situation is different and the below steps are simply suggestions. If you are purchasing or selling a trademark online, please work with an attorney.