Yeah, I don't think that was too much of a douche bag move. Now if you had advertised the car as having brand new interior or mint condition, or if the buyer had asked you if the pieces were new and you said yes, then that would have been a douche move. However, if it looked new and the buyer found it acceptable enough, that's fine. I'm also certain that the plasti dipped interior wasn't what sold the car anyways. Plus, he must've been either a new BMW owner or at least a new e46 owner. Any previous owner of an e46 would have found those pieces in that new of a condition would've looked it over more carefully. Had I been the buyer, if it looked good, I would've been fine.
One more thing, had you coated the plasti dipped with a matte clear spray, it would've been much more durable than even the OEM material with no sacrifice in texture or anything, at least from my experience. The trim outside aluminum trim moldings on my car were both plast dipped, but the driver side was given the matte clear, not the passenger. The driver side shows nearly no degradation of the material after 4 months, and I wash my car on average once a week, mainly hand washing but when I'm lazy, machine wash. The passenger side was done about a month after the driver side with no clear and its slowly starting to wear off, the aluminum rearing its ugly head. I will redo the passenger side, this time with matte clear.