being someone who has stuck himself with intramuscular needles for about 5 years and even reused needles many times, believe me when I say getting an infection from dermarollers is really highly unlikely, especially since you're only breaking the dermis (think about how many times you've scraped or cut yourself and did absolutely nothing for it other than maybe a plaster/band-aid and nothing happened - these kinds of wounds have massively more exposure than the tiny ones created by dermarolling). You have to be a really dirty bastard or have a compromised immune system to get an infection
in fact that was pretty much the finding of a review on pre injection sterilization procedures - they found most of the sh*t they did with disinfectants did almost nothing that simply washing the skin didn't do (there was still bacteria present on the skin in all cases) so they came to the conclusion that the presence of bacteria prior to breaking the dermis was practically unavoidable and the cause of infections was probably related to immune compromise in individuals rather than poor procedure)