Thanks! To answer your question I try to make sure that the offer is not too low. So I also check amazon, google, etc., for pricing to make sure that my offer is fair(ish). And I'll either make the Offer or relay in the Ask.
Usually, on "Firm" prices I just let them know that "x" is what I am willing to spend and they have a backup deal on the table. I live in WA and the app allows me to place a "Hold" on the funds I am offering for the item so the seller knows its a legitimate offer. I can't do it with "Firm" offers but it really gives my other offers some weight since the seller knows money is literally waiting for them!