I'm happy to pay for the skilled labour of bike servicing and parts fitting. I wish bike stores would acknowledge that that's what I'm paying them for and fit parts that I've bought myself (probably off Amazon), charging a fair price that lets them make a reasonable overall margin. Don't try to keep a parts selection instore, use the space for the mechanics and just those parts that people might need urgently (i.e. some pedals that are good enough to ride home on if you broke off your current pedals - if I'm upgrading to fancy pedals I'm going to want to choose them specifically).