And what do you mean "dispensable AGI"? No, buying stock doesn't affect your AGI or lower your taxes. Unless you made a deductible contribution to a Traditional IRA account.
When you sell the stock you will have a gain or loss which will affect your taxes. If you hold the stock for more than 1 year it will be a long term sale and get taxed at capital gains rate. If you sell it for a loss you can deduct the loss from your AGI with limitations.