Make sure the business is closed if you want to invite over any friend etc. Sometimes, it glitches and they may show up but leave anyway if you are closed. Don't use move objects to place any items for sale, those will wind up somewhere else on the lot (some things) if you try to restock etc. You can build a separate building and usually they won't go use your stuff if you do that, but may have to use a bathroom. SimWardrobe had a mod to make them shop then leave without hanging out. Vanilla game, they are more likely to hang out and use your stuff if you are out of stock with something they wanted to look at and buy.