Is it not common courtesy to tell people they aren't the first one interested in your item?

I've done this with almost every item I've posted on the FS forums. Some people, unfortunately, do not do the same, annoying to no end. However, not much you can do to enforce this as a rule.

I have had the annoying experience of being the first person to express an interest in an item for sale. My offer was for the full asking price and the seller decided to take another persons bid over mine. There is no implied expectation of a seller having any honor. I will say that most sellers I have done business with do have honor and are respectable and fair to deal with.

I've purchased from several people here and normally I don't get told unless I ask. I don't think that's unusual though I tend to word things very specifically. If I only have questions about the product then I only ask the questions and that's it. If I have a high interest in purchasing then I'll say that along with, "assuming you don't have other offers already." Or I've just asked if there has been interest in the item. There probably are a lot of inconsiderate sellers and buyers out there. (I've had buyers say they're interested and then never respond again.) If you want to know something then ask it outright.

I respond to pretty much ever PM I receive re items I sell.... not that I sell much, I'm more of a hoarder :D

As a buyer or seller, I think it's best to be upfront about expectations, even if it means asking questions you think might be silly.  I've never had a bad online transaction from asking for too much information, but I've had a couple when I didn't ask for enough information from the seller.

If you're willing to close and pay immediately, I can't imagine why you would have an issue with the seller, unless someone local contacts the seller, in which case it's out of your hands.  I've seen a number of sellers say that local buyers get priority, and I'd probably do the same.

Should a second buyer offer to purchase an item I have for sale I always tell them there is someone else interested but it's theirs if I don't receive payment from buyer one in 24 hours. I then PM buyer one and tell them they need to make a decision in 24 hours. Only once that I can remember a buyer not getting back to me with a a yeah-or-nay. I sold the item to buyer two. Turns out buyer one had gone away for the weekend and didn't look at their email or Head-Fi. They were upset but understood they should have acted and not expected me to hold the item. 

RSVP to ever PM, nicely, but in the end money talks, BS walks. Make a decision as a buyer; i.e., sh*t or get off the pot. If someone posts they will take it while the other guy is p**** footing, so long to the p**** footer, because being the p**** footee stinks.

I find this definitely to be the case for everything except for craigslist.  On places like here, I always give priority to the first responder and let other potential buyers know that the sale is pending but will let them know if it falls through.  This is mainly the case since sales aren't completely anonymous and online forum trading like here usually has some form of rep. I may have not done a ton of trades here, but I have on other forums and I almost never ran into problems.  Also, in almost all cases, the first interested party always took the item.

Craigslist is a different story.  When I first started using it I treated it like everything else but I soon found that most people who respond are always looking to haggle to ridiculous price drops, live far away and don't mention it til the last minute, or drop contact randomly for no reason.  Because of this, I actually respond to all emails and take the best offer or the one quickest to actually finish the transaction.  At first, I used to do it first come first serve and it was a disaster.  I think on average the 5-6th to contact me ultimately took the item.

As a seller and a buyer (not much here, more on CL) but I'm pretty good with trading and such.

As a buyer, you want to show that you know more and aren't as desperate for the item then you need to. It's best to force the seller into a corner to have then believe that you're the only one they should sell to.

As a seller, you want to stand firm. Believe in what you have a wait it out.

I mostly buy and sell bicycles on CL.

Here's a short story:

Selling a Carbon Fiber road bike, about $500.

It's a FAIR price. Not great, but not bad also.

I've had 2 people call over it everyday. 1 person a day to come and see it.

I see it as this, if you know your item and it's worth. Don't try to waver.

I had a guy come by and try to talk my price down. Did all the tricks in the book for several days to get it at $400.

Granted, I only paid $60 for the thing and did free work on it.

But, he put up so much of a fight.

Then another guy comes around.

He see's the bike, hands me $500 without riding it. No problems and 3 words shared.

If you know your item is worth your price and people are biting, don't give into temptation. Just stand firm and wait.

Now as a buyer, you should know the person and the item you're buying.

Point out flaws, difficulties and possible upgrades that are on the market. This shatters the buyer's thought to give into temptation.