Sell first, so the funds exist; commit them through an executed purchase agreement and visa-conditioned escrow; then apply. Closing outright before the interview is also acceptable but leaves the couple owning a business they may not be able to run if refused.
Funds, commitment, application, in that order
Hypothetical example: a Rothesay couple has listed their duplex and found a facility, and the seller wants to close within sixty days. The E-2 application cannot succeed until the funds are traced and committed, which means the duplex sale must close first and the proceeds must be in their account. They can then execute the purchase agreement with the price deposited in escrow releasable on visa issuance, which commits the funds without leaving them owning an unoperable business if refused.
The consular interview date depends on post availability, which the seller should be told is outside the couple's control. Once the visa is issued they can be admitted in E-2 status and take over operations. They should not begin managing the facility, even informally, before admission in that status.
Renewal timing later will depend on the enterprise continuing to qualify, so the operating plan's milestones should be tracked from day one.