I don't speak for NameJet and haven't sold any of my domains there, but I know several people who have. There's no secret, you just submit the 15 best domains in the batch you're looking to sell, and if they're a good fit they'll get the full list from you and work out the schedule.
Then you transfer any domains that get back orders before the deadline to the eNom account that is associated with your NameJet account. If you have little to no history with NameJet I think they might even make you transfer everything to eNom before they get listed. The reason you have to transfer the domains is so they can automatically push them to the winner once payment is secured.
You have to look at it from NameJet's perspective though, if they're getting between 10% and 15% and you submit 15 crap domains you're thinking of dropping, it isn't very attractive for them. Let's say three of them get the min $69 bid, the total sales would be $207 and their commission would be $31. It isn't worth all the back and forth explaining the process, working the domains into the schedule, and dealing with payouts and accounting for that.
Just because NameJet sells a lot of expired inventory, many of which go for less than $100, doesn't mean it is a good model to let private sellers list bad domains unless the volume is pretty big. They barely have to lift a finger for the expired inventory, the same is definitely not true for private sellers.
That's also why you'll hear people say: hey, I bought this domain on NameJet but now they say it isn't good enough for their platform, what's up with that? There are so many valid reasons that could happen. First, you may have bought expired inventory, so they didn't "accept" it being on their platform in the first place, it was just there by virtue of it being a partner drop or getting pending delete back orders.
Second, even if your purchase was from a private seller but they reject it for you, there could be a good reason for it. The domain may have been thrown in as part of a much larger portfolio sale of higher quality domains, and they accepted it then to secure the other inventory they really wanted. Take the domain you purchased on NameJet but they subsequently rejected, and submit it along with a portfolio of 14 LLL.com and I bet they don't reject it.
As far as how many it takes, I've heard of people submitting as few as a couple dozen domains and getting approved, but they were high quality, liquid domains. I've heard of people getting in with as little as a hundred middle tier domains.
I would assume if you're just trying to burn down inventory you're planning to drop, you would need to submit 500-1,000 domains, but I really have no idea on this one. It just depends on what you're submitting and if the potential commission is worth their time. If you submit a decent amount of really nice domains, but every single one has a high retail reserve, they'll probably reject that too.