It is prudent to inspect at or before the time of listing your property with a realtor. A timely inspection will provide you with peace of mind and give you one less thing to do during the regular selling process.
Waiting until you have a buyer for your home can potentially delay or prevent a scheduled closing. It can take 14 days to receive a Property Transfer Certificate.
Minor repairs necessitate scheduling with licensed installers and some require permits and this takes time.
A septic system that has been inspected and approved is appealing to the buyer whereas a system that has not yet been inspected and involves unknowns may provide the buyer with some leverage when it comes to negotiations on the price of the home.