50% sell-through? 400% markup? What I think you should do, is just sell me your business and quit before the stress kills you.
But seriously, the problem with the approach is that there is indeed money in very small purchases, but the only way to make it like that is with volume. Plan B is to maximise your profit per customer. You can do this in all kinds of ways, but just off the top of my head, some ideas which you might consider testing:
1) If your averaging say $1 per sale actual in-pocket profit, start offering every customer who buys a set of koala stickers your
Mega Stupid Koala Value Pack which has either a truly ridiculous number of stickers, or some combination of stickers, stamps, posters etc. which is priced with a substantially lower margin, but a nominal profit of $5. Not everyone will take you up on it, but you have an excellent qualified buyer there; it's silly to not at least try to upsell them. Don't mention it in your eBay listings -- offer it privately when confirming the sale. If you get bites at $5 try a $10 package. If that does well try a $15 package, etc. Somewhere in there you'll find a price point where people just love to load up on product, and if you're making $1 per order and upsell even one out of five of those orders at $5 profit on the upsell, you just doubled your profit.
2) Inserts. If you can source something that is tangentially related, even say art supplies or stuffed animals (not sure what kind of stickers you're selling, so I'm just throwing out rough examples of course) then spend a little money on some nice glossy tri-folds and enclose one in each shipment. I would not expect a huge response, but it is something where you can "price up." And if people are buying the stickers for their kids, you know the kids are going to want to take that glossy and start harassing Mom for an early birthday present.
3) Fundraisers. If you're getting any volume at all of customers who are buying stickers in quantity for a school, church, social group etc. put together a price list of just-over-cost stuff and send it to them along with a letter about what a great fundraising tool your products can be.
4) Personalisation. Get some stock photography and vector art, grab your kid or your sister's kid or any kid whose parent is ok with it, and make some photos of the kid with cartoon characters on the moon, or under the sea, or whatever, and once again hit those established customers with the idea of how much better their lives will be if they order some custom stickers of their kids, pets, etc. I am not 100% on this but I think Zazzle has stickers available that can be customised like this. A pricery option, but again the object is not to cash on a single sale, but to provide many opportunities to add to your bottom line on an existing profitable customer.
Hope this makes sense. The most important thing that you bear in mind right now is that, based on your raw sales data, you have found a winner! Now you should be focusing on expanding the availability and choices, increasing your profit per customer, and -- also crucial -- making it super easy for your customers to tell their friends about you. If you don't have a website, slap yourself hard and then set one up, even if it's one relevant image and a link to your eBay profile. Get a super easy to remember and spell domain name -- if your stickers are mainly kid-oriented, you have another advantage here since your domain name can be any stupid crap you want, as long as it's pronounceable.