There's a very slight difference in meaning.
"I sell items to be offered to the dead" means that the PURPOSE of the items is for people to use them as offerings to the dead. They are made, or intended, "to be offered to the dead." The speaker is selling them with that purpose.
"I sell the items that are offered to the dead" means that what people DO with these items is offer them to the dead. It doesn't necessarily mean that the items were made, or sold, with that purpose.
Also, the saying "the items" rather than just "items" suggests that there are some specific types of items that are offered to the dead, and this speaker sells ALL of those types of items. (The speaker doesn't just sell /some/ items that people offer to the dead. The speaker sells THE items that people offer to the dead. This suggests that there are not any OTHER items that people usually offer, besides the ones that this person sells.)
The difference in meaning is small, though, so they're interchangeable in most cases. Both sentences suggest that the speaker is a person who sells a specific kind of items, and these kinds of items are used as offerings to the dead.