@KeegansCreation @Cat @Mariholly - You never know what features, style, and art a creator has and, therefore, you never know how many have ideas or plans to be able to use such a “time or quantity” limitation. As you said Keegan, since there are thousands of designers, this feature could (or literally would be) something that differentiates the desirability of one product that is always available vs. a product that you have to buy now or soon and miss out (hence the ‘fear of missing out’ psychology of marketing). Further, since things that may never (or randomly) come back, many time it can be sold for more. Additionally, sometimes people simply like the excitement of something that is going to “go away” (and maybe or for certain), and it’s future availability is uncertain or a fact. Ie, Disney sold a ton of VHS video (back in the 90’s) when they would put some/many video titles “back in the vault” and never sell them for 1-3 years, not only creating higher sales when the customer had the chance, but priced them higher than most video and usually had a nicer case.
I know we are not Disney, but if the public discovers something here in Zazzle, there is always the chance (if Zazzle give the the options to choose and adjust) if a product is sold in a limited quantity or time, a designer can try it out and see what the public reaction is in terms of what you can demand and adjust either time (“only for 30 days”) or quantity (“only selling 50” and available until sold out).
Cat, if the algorithm is something you want to stimulate, imagine the “rush” or FOMO (fear of missing out) and/or the novelty when you use Social Media to push your selected “limited” product, the flurry that (sometimes/often) can happen from people either becoming very curious and click your link and/or share your links (wherever) and draw in even more people, well, activity is activity. The algorithm sees, literally, activity and since we’ve had-what-we-had, would you or anyone not want to be able to have this option available to try now and then to see if it works (and there’s always luck, timing, pricing, and competition)? Right? 
Anything different we can do to stand out from the sheer volume of creators here is a chance for more sales, more awareness, and the curiosity or excitement factor is a tricky “button to push” in people’s minds. This features at the least gives us a chance.
As Keegan said, a customer could move onto the next design. So either they didn’t like what you present for sale, the price was too high, they didn’t care as they were looking for something else, or, maybe, they may share it because the opportunity is limited on an item (news travels fast) and people crave novelty. And Zazzle is nothing if not novelty.
Not saying this is perfect, but, it could, just maybe, make something feel special because a customer may in fact miss out if they move on or wait too late. You may pay for advertisement. Why not an idea for free that may (sometimes) work? You share you links? Why not an idea for free that may make people share more often? Ever see a crowd check out a car accident? What about when a group of people start yelling loudly? Novelty, curiosity, and rarity are marketing ideas that we want to be able to use here in Zazzle.
Zazzle can give us the option and we can choose, product by product, to activate or remove the feature (time limit or quantity limit) to help them and us sell more and make more money.
Imagine a Zazzle competitor having the same features as us, but they have the added feature of time or quantity limitations to stimulate the above motivations for sales from customers and we did not?
Look online at other merchants in other places that sell time or quantity limitations. Many customer come back again and again to see “what’s next?” or “what’s new this time?”. That sounds like something the algorithm and customers could respond from if this increases the activity of link sharing, repeat viewing, increased viewing, and the more people that come check out what is happening, the more sales we could have.
Hope this makes the idea sound more clear and interesting.
It would be worth having it available to our business success.