Sharp Elbows necessary in this hobby?

[mintacular]

Hi, this seems to have happened to me a couple times recently and it kind of bothers me. I was at a show and was in the process of making a deal for a largish vintage lot. Well I had the first box set aside that we came to terms on and we were then proceeding to the next "sister" lot. Another guy approaches quickly behind me and is staring at me with his beady eyes, etc. as he sees my one box of vintage cards. Well, he then asks the dealer "Do you sell any other vintage?" and the dealer stops talking to me and tells him that I bought the one box and we were in the process of negotiating for the other one or something to that effect. At this point I kinda look at the guy with a "back off buddy" look and continue with the dealer for the other group of cards. To make a long story short, the situation kind of became uncomfortable and also maybe drove the $ up a bit as the dealer (he sells mostly newer stuff and other items) realized there was other interest in these cards.

Anyways, has this situation ever happened to you? Isn't it poor etiquette to barge in if you are at the table first? Or is that just part of the game? Your thoughts are appreciated...