Oops - got on a phone call.
Gross Revenue
I don't follow this - you'd have $800 of gross revenue and an unspecified residual value of unsold inventory, right? I don't get where the $200 comes in, other than as (200) (i.e., negative 200) as "revenue less costs of goods sold".
The difficult piece here is "basis" - i.e., what's the cost basis for what you're selling. If you're selling (and buying) singles, that's easy. If you're selling (and/or buying) sealed inventory, breaking it yourself and then selling a portion, that's the problem as you'd need to track (I think) both the value of the sold items (which would easily translate from their sold price) plus the residual value of all of the cards that you didn't sell, which would require some sort of reasonable valuation that would potentially be subject to audit.
Now you'd likely have to allocate (near) zero value for e.g., commons or filler cards like the base set in SPA, so you're left with the "valuable" cards that you're more likely to be keeping. So not sure you could claim a loss in our example, I don't think, if you sold two cards for $800 but kept a bunch of other cards that were worth $500 (as you'd have a paper gain of $300). Complicating matters is that prices move frequently...
But you'd need to track all of it so you have a sense of what your cost basis is in the cards you've sold as compared to the cards you're retaining for sale later (or for keeping in your PC).
No matter what you do, your basis will not increase for any of the cards from the sealed case beyond the original case purchase price of $1000, ignoring the cost/value of all of the deductions that have been specified above.
I'm sure there are "market" or "reasonable basis" approaches that are taken for people who face these issues regularly, particularly where the value of what you're opening is unknown and can only be ascertained from the sealed product purchase price (which as we all know sometimes has no bearing on the value of the opened product, for better or worse).
In case it's helpful, I'm almost certain than under US rules, gambling gains can be offset by gambling losses. Not saying that opening cases is gambling per se but that may be a useful proxy for coming to a reasonable position by figuring out the value of the cards that are kept (i.e., unsold) as compared to those that are sold, all in relation to the cost basis of the case as a whole.
Fred Bear's caveat is 100% the right one though - don't rely on a message board for tax advice. EVER! Speak to your accountant. 
