yes dealers have scales.
but they don't carry them around on the street, they use them at home to make pre packaged bags that they then take out and offer for sale
if you're buying weight (more than the amounts they commonly carry) they'll weigh it out ahead of time too, nobody wants to bust out the digital scales and start measuring coke by the miligram while out on the street.
you as the buyer don't bring your scale to the deal either. what you do is you buy your goods as agreed upon and GTFO as fast as you can. then later, when you're safe at home, that's when you take your own scale and check if the weight matches what you paid for.