Learn English – “being” vs “were” after “due to”

Revenue declined due to lesser units of property being sold in 2013.

Is grammatically correct. "being sold in 2013" is a participle phrase that acts as an adjective to "units of property".

You can use the participle (being) as and adjective, but not the conjugated^[1] verb form (were).

A point about "lesser": this seems to indicate that the units of property that were being sold (adding "that" would make the second sentence correct!) are of lower quality compared to others. If you simply refer to the number of units that were sold, I would use less.

A bit more details about why this sentence is wrong:

*Revenue declined due to lesser units of property were sold in 2013.

From here (Emphasis mine):

"Due to" is very similar to "because of." It gives a reason for why something is true. You can use "due to" with phrases and single words, but you can't use "due to" with a clause.

Now "units of property were sold in 2013" is a clause, and should not be used after due to.

You could however substitute because for due to, and the sentence would be fine, as J.R. pointed out. This is because because can join two clauses.

^[1] Conjugated, not declined, of course. Thank you @SnailPlane, and my apologies for confusing anyone, including @Pupu!