^^ Good advise. If its a decent shop, they shouldn't mind, and be happy to get someone on a bike. At our shop, we obviously would rather you bought from us, but are happy to help no matter what. We may give you some ribbing, and a joking suggestion to buy lunch, or drinks, or wash the windows...lol.
We have done this for many many people looking to buy a used bike online or whatever. Its safer to meet in a public place anyway.
A word of advise, if you do go this route, and buy the used bike from someone besides the shop, throw them a bone and have them do a tune up, and maybe pick up a few things while there, like gloves or whatever. And keep going back to them for service and help.
And one thats a biggy. Some shops will do the once over and fit thing for free, expecting you to come back in the future. STILL OFFER THEM SOMETHING. Even if its a hand around the shop. Yesterday some guy offered to take the trash to the dumpster for us after we did a little 2 buck brake adjustment. It was a nice quester, and much appreciated as we are swamped with spring tune ups.