They can be any form of upgrade, credit card, sms, pbp, pbc, but mostly are sms/pbp pins. Since most pins are gained illegally through pin.lines, sythe could be held liable for it. So now they're called 'upgrades'
Most of the time the person upgrading either has some one give them a large supply of upgrades for a cheap price or gets them for free them self then makes a small profit for upgrading others.
most people who sell pins cheap obtain them illegally through online pin lines/stolen credit cards/stolen phones/etc. and sell them cheaper because the pins only last a certain while. Most of the time you won't get caught by jagex, but if you do they might put negative membership on your account