I used to find that old Shia LaBeouf video of him screaming at you oddly motivational, more particularly when I was opening a new shop and had an incredible amount of work to do to get listing up. 
It was a joke, but it was just the external pep talk that appealed to me at the time, and the last line always got me, lol.
There's also the "no more zero days" approach to things (if you click the link, read the first reply to the thread, not the OP) - it's talking about it in an approach to life in general way, but the basic jist of it is never have a day where you don't do at least one thing towards your current goals, even if it's just a few minutes, .
Motivation has a snowball effect a lot of the time - though I prefer to be determined* to do at least 5 mins of a task, if you don't do any more than that, at least you got something done, but the vast majority of the time, you'll do more, because you're already there and what's another 5 minutes?
*not motivated - determination is better, IMHO, it is a conscious decision, while motivation for me always seems like waiting to feel like doing something.
Music never hurts. 
Definitely a little each day.
Or perhaps list in 5 -10 card lots and mention in the listings buyers can make up their own lot if they message you.
I've heard chocolate (or some other favourite treat) can work well when it comes to getting through the legwork.
Put a freddo frog (or any small thing that has a wrapper) in between every X number of cards (preferably more than 2 ) , and as you work through the pile, you get to treat yo'selfโข.
