I cannot think of a legitimate reason why someone would do this.

If you will not be active on the forum to check your thread very often, you can provide alternative contact methods and a note on your OP that you may not quickly see any replies.

I have seen many people who are transparently scamming do exactly this, sometimes with fake reviews. The warning shown on OPs who have negative trust shown to guests is a step in the right direction, but I think it should be a rule that active sales threads that are locked are not allowed. This should not prevent locking a thread while a sale is "paused",  "on hold", or "temporarily out of inventory".

Maybe a good solution would be in order to lock a thread, the OP must select a choice of if his transaction is "finished" or if he "intends to resume the transaction at a later time". An "automod" message (similar to what is used on Reddit) could be prominently displayed.