How I would personally deal with this would depend on what the actual item is, and the specific nature of the complaint (if it is specific, a general dissatisfaction with product value is quite vague, and even I - as a buyer - sometimes think the price I paid for something was a bit more than I would value it, the only difference is I wouldn't message a seller to say so, I just either wouldn't buy it again, or not for that price if I did...)
The simplest way to respond is to mention any of your store policies that may apply, whether or not the terms have been met, so they have some understanding of what they are, that you're firm in applying them, and more importantly whether or not they're eligible for something to be done.
Re: the feedback, one thing to consider is that many buyers will leave it as soon as they receive something, because they've got what they ordered and so forth, and then sometime later they'll actually open, inspect or use the product and find it doesn't quite meet expectations or do what it's supposed to. If it's the kind of product (or issue) where it's a fault that might not be immediately apparent on receipt, then I would give the buyer the benefit of the doubt and request some further information, then handle it the same way I would any other similar issue, with or without FB having already been left.
If they are generally just unhappy with the overall product and think they paid too much, you have a couple of options depending on your return policy, the current condition of the item, and/or how far you're prepared to go to 'satisfy the customer' as it were. (eg although I allow change of mind returns, there's [officially] a 7 day window after the buyer receives the item for them to request a return. I have a couple of other stipulations re: condition of the item, but as long as they're met, I would probably accept a return outside of the 7 day window, the buyer is responsible for return post and they get a refund of the item price. If it's been several weeks since the purchase, though, I wouldn't accept a change of mind return, in which case the response needs to be kept polite and simple - eg 'unfortunately as it's now X days/weeks since purchase, the item is no longer eligible to be returned', I try to end on a positive note, either by offering alternative options if I can, or if not, simply by saying that I'm happy to help with anything else, in some cases I've granted permission for my images and description to be used if they wanted to onsell the item).
