Propose a reasonable solution to the co-owner. Offer to buy or sell the partial interest for a reasonable and fair market value, or to list the property for sale at a reasonable and fair market value. If either party is unreasonable, a court will take that into consideration when the proceeds are divided.
Put it in writing.
If your efforts at being reasonable are not being heard and the other party is being unreasonable, put it all in writing, do not be confrontational or argumentative. Keep copies for a judge to consider later.
Hire an experienced attorney
Do not wait too long. If the co-owner is not cooperating, hire an experienced real estate attorney. An experienced real estate attorney will tell you there is not any reasonable way for the opposing party to an action for partition, and that there will be a good chance some or all of your costs will be taken out of the unreasonable person's share of the proceeds.
File a lawsuit for partition.
Lawsuits of this nature in Los Angeles County may take a year to a year and a half, and many if not most Judges are familiar with partition cases and will early on figure out who is being unreasonable. Since the Judge must appoint a referee to conduct the partition sale, there will be additional costs (usually taken from the proceeds) which, most often, will be charged to the co-owner who unreasonably refused to cooperate from the beginning.