Why don’t I send my book to you
Although you often urge me to?
The reason’s good, for if I did
You’d send me yours — which God forbid!

[Non donem tibi cur meos libellos
Oranti totiens et exigenti,
Miraris, Theodore? Magna causa est:
Dones tu mihi ne tuos libellos.]

Martial (AD c.39-c.103) Spanish Roman poet, satirist, epigrammatist [Marcus Valerius Martialis]

Epigrams [Epigrammata], Book 5, epigram 73 (5.73) (AD 90) [tr. Pott & Wright (1921), “Return Favours”]
    (Source)

Compare to Epigram 7.3. (Source (Latin)). Alternate translations:

Thou wonder'st, Theodore, why I
Entreated oft and urgently
Refuse to send my Bookes to thee;
I feare thou 'lt send thine owne to me.
[tr. May (1629), 5.74]

Why I dole thee not my pieces,
Theodore, thou may'st devine.
Yet thy wond'ring zeal increases:
Lest thou should'st redole me thine.
[tr. Elphinston (1782), Book 3, ep. 48]

"Why ne'er to me," the Laureat cries,
"Are poet Paulo's verses sent?"
"For fear," the tuneful rogue replies,
"You should return the compliment."
[tr. Hodgson (c. 1810)]

I give thee, friend, no works of mine,
For fear you should return me thine.
[tr. Lamb (1821)]

Do you wonder for what reason, Theodorus, notwithstanding your frequent requests and importunities, I have never presented you with my works? I have an excellent reason; it is lest you should present me with yours.
[tr. Bohn's Classical (1859)]

Though it's true, Theodorus, you frequently pray
For my book in a flattering tone,
No wonder I'm slow; I've good cause for delay
In my fear you'd then send me your own.
[tr. Nixon (1911), "Vendetta"]

Why don't I give you my works, although so often you beseech me for them, and press me? Do you wonder, Theodorus? There is great reason: that you may not give me your works.
[tr. Ker (1919)]

Do you wonder, Theodorus, why I don't give you my little books for all that you beg and demand them so often? For an excellent reason: I don't want you giving me your little books.
[tr. Shackleton Bailey (1993)]

Ted, don't give me pleading looks,
And beg I send you all my books,
Your ask comes with a healthy fee:
You'll then send all of yours to me!
[tr. Ericsson (1995)]

Why, Theodorus, don't I send my books, though you demand and plead repeatedly? My answer's good: so you won't give me yours to read.
[tr. McLean (2014)]

You ask my verse, so here. This evens scores:
I had kept mine in hopes you would keep yours.
[tr. Young]

You wonder why I never ask you if you’ve read my book?
I’m not one of those narcissistic bores
who fishes around for praise with such a thinly baited hook.
Besides, I’m worried you’ll ask if I’ve read yours.
[tr. Clark, "A Good Reason"]