Get in touch with the other person in game and set all the parameters for finding the item (ex: sword, level 30 to 30, enhanced effect min: 100) then once you're all set and ready to buy you message the seller and they put it up on the market for the agreed price. As soon as it's up they'll message you back and then you hit search and buy it. If you're quick about it then you shouldn't have a problem with someone else buying it
And make sure you've read this:
http://ladderslasher.d2jsp.org/index.php?g=1#Marketplace