I'm not so sure he's going to make a profit, but very few dealers today - internet included - buy modern issues in quantity. It's just too expensive.
The result is that many of these modern stamps are starting to rise in value as collectors try to find them.
It really depends on what he means by "modern" issues. If he's talking since 2000 and later, he can probably offer them up on
ebay and may make a few dollars back of what he spent.
If he offers them to a dealer, he's going to lose 30-50% of what he paid.
The best bet would be to sell them one sheet per listing, at 90% of face, on
ebay and see what happens with the first fifty sheets he puts up. If there's no interest or he's not getting what he's asking, then look to sell them discounted to a dealer.
Just my recommendations...