You don't have to list something as make an offer unless they changed it. I get kinda annoyed when people list as make an offer and then you offer something reasonable (10% off) and they decline it. Just list it at a non-offer price then...
I messaged a guy my buddy has purchased from on discogs about some MoFis he had listed and asked for pictures of the sleeves since they were marked VG (conservatively graded). The guy emailed me pics and chatted back and forth and he ended up giving me a pretty steep discount on two titles. I paid paypal G&S direct to him instead. Worked out for both of us.
I suppose when I meant toggle, I meant in terms of making it more developed in terms of options. Allow for counter offers, allow for sellers to let people make an offer after a reject, allow for contingent offers (I'll buy x and y from your store for $xx) etc. If a seller wants to one-and-done it I'm 100% good with that, I just feel like it'd be better if the option were there for a seller to send a counter offer back or for the seller to decline but to be open to future offers from the same person. Sellers, as you say, aren't required to enable make offer at all (nor should they).
Edit: Of course, if offers aren't open, I just assume the price is set. That seems reasonable.