As someone who has no knowledge of this space - what does it mean inventory being bought out? You can no longer place your ad on that subreddit anymore? Why would this happen?
Say the subreddit generally only get 5,000 page views a month, with one ad space per page. Once someone purchases those 5,000, it's gone -- they can't sell the inventory to someone else because it's all bough. Often too traffic will fluxate; so, for example, they may have estimated 5,000, but only 2,000 page views (and thus ad impressions, since one for one in this example) come in that month -- if someone bought 5,000, they'll have to stick around for 2 1/2 months, blocking other advertisers for that time. And I suspect that on many of these subreddits having even 500 impressions available is overreaching. That's a bit simplified but hope it helps.