There are a couple of ways to do this. If you haven't yet collected all the paintings, then grab two of them by following my collectibles guide in the walkthrough, and sell them both at the same time to the art dealer.
Alternatively, you're going to have to upgrade your inventory and go for the big hauls from many houses without getting killed or arrested in between. Gold bars ($100), diamond rings ($75) and watches, necklaces and bracelets ($25-$50) are what you're looking for.
Check your inventory and calculate the worth. When it's over $1000 go to Reese Pawn and hit
to sell everything at once