Good points by WBC on being self employed. Incorporating a company is the best way to go and also limit liability and probably taxes.
We have a member, Bill Harrison of S and W Parts on the forum that has S and W Parts as a business selling parts and making repairs for model trains. He may be a good reference and source on the business of repairing model trains. S and W Parts is a forum sponsor and advertises in OGR magazine.
He is S AND W here and at trains@sandwparts.com and WWW.SANDPARTS.COM
He is also a LIONEL PARTS DEALER AS WELL AS LIONEL SERVICE STATION 91
Charlie