A retrade is a buyer's request to lower the price after the letter of intent is signed and exclusivity granted, usually because of something diligence found.
In a sale
Because exclusivity is when the buyer has no competition and the founder has stopped talking to anyone else, so an issue found then is negotiated with one buyer instead of priced by several. The same issue disclosed before offers is part of the business the buyers bid on. The defense is built before launch: each issue fixed, or disclosed on the founder's terms, before the buyer finds it.
Where it comes up
The Go-To-Market Analysis (GTMA) builds the file a buyer would build on your business, before a buyer does.