as someone who does a little spannering for mates for beer there are some serious concerns for the home mechanic setup.
income – declarations/tax etc
insurance – a) against theft of someone elses property while in my possession and b) against legal action when i slip up
planning permission – change of use, residential dwelling for business use
there is no mark-up on parts, nobody would pay it when they can place that same order online, but they will want you to order it, which puts you in the “seller” position for anything “warranty/returns” short of drawing up a contract
advice is free, mistakes are costly and that risk sits with the mechanic, and, in any customer service environment “the customer is always right likely to complain and return to bite you in the ass”
so add that little lot together and come up with a suitable hourly rate?
EDIT – from the link above “•Gear Tune Up £20.00”, so whats that £60/hr