Seldomly is a superfluous word. Seldom is already an adverb, so adding the adverbial -ly does not change its meaning.
Using seldomly is not a serious error, however, as only strict followers of English usage have a problem with it.
Examples
In each of these examples, seldomly could be changed to seldom:
Deblinger says children seldomly come forward to report their abuse … [ABC Local]
When talking Michigan football, the word “inexperienced” is seldomly used. [Collegiate Times]
The characters look so poorly aliased and their dialogue matches their mouths so seldomly that the game looks like it was created by a ’80s-era Japanese studio. [Auburn Pub]
