I have several friends who do it, and I'll even go take a look on our local site when I'm bored and wanting something to do! I say its perfectly normal behaviour... and if something good comes out of it, well that just makes it better haha!
As long as you're not harassing the seller when you have no intent of buying, I'd say you're par for the course as far as horse people go!
Totally agree with this though. If you're just innocently looking, keep it at that :) I just sold my gelding after a long time on the market, due to time wasters, all wanting a show horse with expressive movement, but not actually wanting to experience such movement in the saddle. I posted a video of his trot and canter up on the ad and that seemed to stem the wasters, but yeah, from a sellers perspective, if you're just browsing, don't contact haha!