I know absolutely nothing about PC repair business.
But I would make a suggestion that may help you both in respect to deveolping a customer base and dealing with problem customers.
Reconsider having computer gismos to sell.
Not for the money in mark-up but for these reasons:
People are often afraid of things they are unwilling to admit to even themselves, let alone somebody else.
Having the latest Video card they could open a conversation about--or just any decent one--a Cd-RW whose operation you might help explain not that they don't completely understand it already, maybe a sound card or two for people "who just dropped in to see if they could help a friend with his sound system" might take them off the hook enough to be willing to engage your services.
I gather you don't want to get stuck with unsold inventory so we are talking about a few mainstream items here, and their function is more psychological than economic as such.
The other advantage to engaging people in a conversation about some computer item is that it gives you some sense as to "Where their head is at" and whether this is someone you want to deal with
If not, as bartenders say, "They don't get a major reception". Why do you think a bar door person will engage you in a conversation before you go into a half-empty bar. To see if you are "tight" and to see if you'll be a PITA.
Hence, "selling" some choice items--or at least having them around-- helps to deal with your concern about "problem customers". While nobody can predict how someone will turnout to be, your "street sense" will help you out here. In time you'll probably develop a better sense of whose gonna more pain than he's worth.
Good Luck from the
D
G