What is the availability of the complete sets Wizards of the Coast is apparently selling?
The picture, and eBay auction you link to is for a set that Wizards sends out as part of Magic Online set redemption. The way this works is if you have all of the cards in a specific set, and it is before the cutoff date for redeeming that set, you can have Wizards remove the set of cards from your MTGO account and mail you a set of physical cards. This video shows the boxes from a set redemption, and what the contents of that boxes look like. These sets are never sold by Wizards, though as this auction shows they can be resold by third parties.

Wizards of the Coast is not selling complete sets of cards. Third party vendors such as Star City Games and Channel Fireball however do. These third party vendors offer the complete sets by opening packs and putting the sets together themselves.

These vendors usually offer these products (the larger ones pretty much always do, smaller ones may or may not, it depends on if they think they can afford to or not), as for how long they offer it it depends entirely on how easy it is to keep in stock, and restock when they sell out. This means in general you can find newer sets for sale, but as you go farther back it is less likely that they will have a complete set in stock.