What would be the net effect if the non-Jewish person to whom I have sold my chometz for Pesach converts to Judaism over Pesach?
From various Mishnayot (see below) we see that a Ger's possessions seem to continue to belong to him after he converts.
In that case, the moment your buyer converts, he owns Chametz on Pessach - Chametz from which everybody is now forbidden to derive benefit from - similar to any Chametz a Jew owns on Pessach.
Result: He would not be allowed to sell it back; he would have a Mitzva to burn it ASAP.
נָכְרִי שֶׁקָּצַר אֶת שָׂדֵהוּ וְאַחַר כָּךְ נִתְגַּיֵּר, פָּטוּר מִן הַלֶּקֶט וּמִן הַשִּׁכְחָה וּמִן הַפֵּאָה. רַבִּי יְהוּדָה מְחַיֵּב בְּשִׁכְחָה, שֶׁאֵין הַשִּׁכְחָה אֶלָּא בִשְׁעַת הָעִמּוּר:
We see that the crop is his; and his ownership carries over from before his conversion.
גֵּר שֶׁנִּתְגַּיֵּר וְהָיְתָה לוֹ עִסָּה, נַעֲשֵׂית עַד שֶׁלֹּא נִתְגַּיֵּר, פָּטוּר, וּמִשֶּׁנִּתְגַיֵּר, חַיָּב. וְאִם סָפֵק, חַיָּב, וְאֵין חַיָּבִין עָלֶיהָ חֹמֶשׁ. רַבִּי עֲקִיבָא אוֹמֵר, הַכֹּל הוֹלֵךְ אַחַר הַקְּרִימָה בַתַּנּוּר :
Here it's even clearer that the dough belongs to him even after his conversion.
:גֵּר שֶׁנִּתְגַּיֵּר וְהָיְתָה לוֹ פָרָה, נִשְׁחֲטָה עַד שֶׁלֹּא נִתְגַּיֵּר, פָּטוּר. מִשֶּׁנִּתְגַּיֵּר, חַיָּב. סָפֵק, פָּטוּר
Here again we see that his ownership of the cow continues through his conversion; freeing/obligating him with Matnot Kehuna (זְּרוֹעַ לְּחָיַיִם קֵּבָה ).