Preventing transactions between two third parties (i.e. ebay) is economically similar to refusing to do business with another party. Except that if your in position to do the latter, you're obviously a platform for others, and thus you have a lot more unilateral power to prevent transactions between arbitrary third parties. Which is probably a lot more economic transactions than the number of transactions you engage in yourself. It's like leverage.

So the harm seems proportional to the harm done by refusing to bake that wedding cake for the gay couple.