I can’t tell you how many times I’ve been inside a listing that looked clean enough and well-maintained until we got to the furnace. And then we found a filter so clogged full of lint, fur, dust and debris that we wanted to run outside into the fresh air.
You should change the filter every few months, more often if you’re doing construction or renos. If you don’t know that, or know and don’t do it, it looks like you don’t know the first thing about home maintenance. That makes the prospective buyer wonder what else has been left unattended.
A new filter might cost twenty or thirty bucks for a good one (I wouldn’t buy the really cheap ones; they’re not very effective). Besides, your furnace works like an extra set of lungs. Do you really want to be breathing in dirty air?
There’s another good reason to change the filter too: a dirty furnace has to work harder which means it’s costing you more money. Do yourself a favour and buy a new filter. If the buyer doesn’t notice, believe me, the home inspector will.