The whole idea of buying online from a person rather than a company—whether it's Reverb, ebay, craigslist or here—is a complete Wild West situation, as far as I'm concerned. From my point of view, as both buyer and seller, if both parties get what they want and no one gets stabbed, the transaction was a smashing success, communication etiquette be damned.
On the TB classifieds, I've sent PMs with, basically, "I'm ready to pay your asking; let's do this" and not heard back for weeks and weeks. (Yes, it's still for sale.) Frustrating? Perhaps, but nobody owes me anything. Maybe the seller doesn't like my screen name or avatar. Whatever; I'm not holding anything against him.
So, specifically regarding the OP's annoyance: I've found that if I see something for sale and "Make Offer" is available, I'll almost always offer something low, expecting to haggle. The offer is rarely my "best offer"; it's me trying to get the biggest price break possible on something I'm probably willing to pay more for. This doesn't seem cool, but I am a beast the market has made. Likewise, when I list something for sale and enable "Best Offer," I have to mark it up (especially on craigslist) because I know people are going to come in under asking. It's kind of an annoying racket, all of it, and I'm a bad negotiator and I hate it.
But to get right down to the point: it me. Not specifically regarding your Ibanez, but...the internet depersonalizes everything, and when a number comes up on my computer, it depends on how I'm feeling that day. Where I'm at. Did I just get hit with a huge tuition or real estate tax bill? Am I feeling like GAS has got the better of me lately? I'm more likely to take the offer. Has this shopper started off with some offers so low that they were auto-rejected, and he keeps inching up into almost-reasonable territory? LOL...F that guy. So if you offer $600 but your next offer is $650 and I have no idea who you are, then I also have no idea what the ax is worth to you, and I start to think "Is it worth it to me to box this up, fork over Reverb fees and go to the UPS store today (because hell yes, I'm a fast shipper, y'all)—all for less that I was asking, just because Joe X from Florida or Texas is trying to get what he wants as cheap as possible?" A lot of times I arrive at "Nah, I think I'd rather put in my ear buds and go for a walk in the hills and put this BS behind me." On occasion I actually plug in the instrument in question, play it a little, and decide to hold firm at my asking. Or take it off the market entirely. I'm not a business and I don't care if it frustrates people. There's plenty of Telecasters in the sea, as they say.
I'm most likely to haggle with someone who has sent me a message, expressed interest, maybe asked a question, and I've gotten a sense of who they are and whether they're someone worth dealing with. I once sold a guy a guitar super cheap because he was the leader of a church band for kids. I would not have made that deal for someone who just punched that number into their browser.
Bottom line: I have no regrets regarding pricing. A guy will say "I notice this guitar has been for sale for two years. Do you really want to sell it? What's the lowest you will take for it?"—as if I'm trying to unload a fine instrument ASAP and I don't know how this works. Full disclosure: I have things for sale that I don't actually want to sell, but I know I will one day, and at whatever point between now and that day someone comes along who is willing to pay what the instrument is worth to me, a deal is there for the making.
Actual bottom line: if you love love LOVE what I'm selling and definitely want it, click "Buy It Now," pay my asking and you will get it fast and expertly packed.