Negotiating: I would use your shop experience, about negotiating prices from suppliers down, and maybe with your experience of dealing with bulders and workmen, and negotiating prices. Do they have some specific questions on negotiating?
Influencing/persuading - would talk about finding out what the other party is like, what pushes their buttons etc: i.e potential results, facts and figures, 'expert opinions' etc, and constructing your arguement in the relevant way to that individual.
Key Pefromance Measures - is this wrt to suppliers, or your area's performance?
iF SUPPLIERS, IT WOULD BE ABOUT MEETING CRITICAL DATES, SERICE LEVELS (DELIVering stock when you need it, on time),qulity standards, meeting delivery requirements (i.e complying with company supplier manual if applicable)Using/complying with sytems required by the business, i'e EDI ordering etc.
If for your area, it weould be sales budget, profit/margin target, cost controls, stock levels, stock avalable on time for re-ranging in store etc.
Other pluses for you IMO are maturity, people skills, interactions with people. Re graduates, you have much more expreience of differnet ways of influencing people, juggling many tasks/demands, managing your time, doing things to a particular standard.
You have also run a small business so you undersand about getting to know what your cusotmer wants, analysing your market/customer base. Ranging/renge selection, pricing, negotating.
Managin stock levels, in store merchadsing, need to manage/amintain display in store, pricing, promotions, cashflow, managing a supplier base, managing and delivering on projetcs etc etc.
sorry for appalling typing and spelling, need to dash, but wanted to help if I could. Good luck!