The truth-maker of 'Tom sits' cannot be Tom. Otherwise it would also be the truth-maker of 'Tom stands' which is the logical contrary of the first sentence. And that won't do, as London Ed appreciates. But now what about 'Tom exists'? This too is a contingent sentence, and so it too needs a truth-maker. I say the truth-maker is Tom. The truth-maker of 'Tom sits' is a fact, the fact of Tom's being seated. This fact is a complex having Tom himself and the property of being seated as constitutents. (Let's not worry about what holds these constituents together!) The truth-maker of 'Tom exists,' however, is not a fact having Tom and the property of existence as constituents.
Why the asymmetry? Because existence is not a property in the same sense of 'property' in which being-seated is a property. I won't repeat the many arguments I have given on this blog and in my articles and book.
But suppose you, like Ed, see symmetry where I see asymmetry. You think that the truth-maker of 'Tom exists' is the fact of Tom's existence, or the fact of Tom's existing. Call this truth-making fact T. Since T exists, and exists contingently, 'T exists' needs a truth-maker. I am willing to concede that a vicious infinite regress then arises, though the matter is not entirely clear.
But what does this show? I say it shows that the assumption that existence is a property is mistaken.
The dialectical situation is this. There are plenty of arguments why existence cannot be a property. And we have good reason to admit truth-makers for contingent truths. So in the case of contingent existential truths like 'Tom exists' we should say that it is the referent of the subject term itself that is the truth-maker.