“Fiddle with” has a similar meaning to “tinker with”: it means to touch something restlessly or nervously, often with the purpose of making improvements; “Messing with” means to meddle or interfere with something, so it does not either express that Nelson is skilled at his hobby, and neither does “Selling,” which comes from the verb “to sell.”
“Repairing,” on the other hand, means to restore something damaged or worn to a good condition, and it is the only word that can change the flavor of the sentence to show that Nelson is quite skilled with small appliances.
