A better comparison may be with craigslist rather than eBay. As a regular scavenger of craigslist, I have observed that when someone relists an expired ad, it uses the same link. I can see this because the link color indicates I have already viewed the ad, yet it appears near the top of the list when sorted chronologically.
I think it is beneficial to use the same link for the same ad because if people thinking about buying the item have placed it in their bookmarks, it will be easier to find.
On the SEO front, as a user I have observed that Google is not very good at removing dead craigslist links, though they have improved. I regularly search for specific items in other craigslist locations using Google and hit removed ads regularly. It seems that if a page is already indexed, it is more likely to appear higher in the results.
Again, looking at craigslist, they do not return any errors when returning to a page that is an expired or removed ad. There is just a message informing the visitor that the ad has either expired, been removed by the author, or flagged for removal by the community. Perhaps your site could inform visitors that the author of the ad has suspended, paused, etc, the ad and it may one day return. Or not.
To answer your question on how eBay currently handles auctions that have ended, the auction is available at its URL for something around 90 days, and is then removed from the site. As an infrequent seller, I have never cared that the closed auction is still there, and I would think that professional sellers would prefer it because it means that they effectively have a greater presence in Google searches. When a consumer finds the closed auction, if they are anything like me, they are most likely to first click on the seller's store to see if the seller has another auction for the same item. EBay also informs if the seller listed a new auction with the same or a similar item and includes a link to it.