I think it's a great idea, but you won't resolve the issues
without a specific scenario, and the issues will make it pointless.
For example, say you get a load of subs (500+), people will snap up names quickly and there will be some who never get names from the list as they're always gone. You lose subs as they're
paying for nothing, making it pointless for some, your time, etc etc.
So to resolve such issues, I suggest the following (ideas):
Limit the max subscribers to 250 users at any one time
This makes it
exclusive which benefits you for ease of list management, and subscribers as they all get a better chance of grabbing a domain from the 250. If you have (eg) 1000 subscribers, you'll just have a lot of unhappy people always complaining there are never any left, and they'll unsubscribe anyway,. so likely the list would
naturally reduce to around 250 limit anyway (ish).
If any unsubscribe, advertise the free space(s).
This also means you can charge
more money for the service. This makes it a viable model for you and subscribers, as your time is paid for to a satisfactory level, and subscribers get a good value service as they can actually get use (buy domains) from a service they pay for.
Limit the max domains from the list per subscriber
eg "only X domains per subscriber", this could mean fiddly time for you checking each time, and also there will be times where a handful of subscribers don't register something, which is a waste for the others who could/would have regged them. So this is something to think about, but not sure it would work.
I wouldn't negate it altogether though, as some subscribers with large investment funds might just scoop the lot up (or a few users grab 70 each etc leaving nothing for everyone else). Perhaps limit to 15 per subscriber. You'd soon find out if someone was regging too many as you'd have a fair number of the 250 subscribers complaining there are never any left.
Free trial, initial changes
You are going to have a fair bit of trial and error throughout the first 5 or more lists, and you cannot give 5 lists away free. You are providing a great product/service for the subscriber fee even at first - they get a great list of cheap names to grab. So personally I wouldn't bother with a free trial. Instead just make it clear the first 5 lists are on a "introductory price" (say 75% the normal price) and make paying subscribers aware that the rules and setup you have at first will be subject to change based on trial and error. eg you limit the list to max 250 subscribers, and every week/month there are (eg) 100 domains not taken, so you can increase the max user base to 350 (or whatever).
I've not proof read or trimmed as I just spilled my thoughts on the page, just ideas