This is quite a simple and to a degree easy and 'old' scam. Its probably been posted before but i couldn't find anything that was reasonably similar after a little search.
Stand is market and say your selling like 27 cps for like 1kk, which is half price on my server, and then when they put up the money put up 27 silver and accept. If they aren't paying attention they'll accept.
Dosen't always work, but i've made quite a bit of money from it. If they start calling you a scammer saying you traded 27 silver instead of 27 CPs just say they are trying to scam you out of another 27 CPs.