If you got to the larger, standard sized crankset, you'll need a new chain and the front derailleur will need to be repositioned and tuned up.
Later you might find you don't like the cassette you've got with the standard size crank, and will want to swap that out too.
So it might end up not being a very good deal, since you'll need to change at least the chain, and might need some work done on the bike.
If you were happy with the compact crank, shop around for a replacement. 50/34 are widely available. It sounds to me like the shop just wants to sell you whatever they happen to have on hand.
If it were me and I weren't in too big a rush, I'd look on eBay for crankarms only, since you already have chainrings and everything else. Often crankarms alone (without chainrings, bolts or bottom bracket) sell pretty cheaply. Most folks are looking for complete sets.
You just need to be sure to get a crank that matches up to your current bottom bracket (EXO or Isis or Octalink, for example) and that has a 110mm diameter bolt circle for the chainrings (all compact cranks do). If it's a fairly common brand/model, you might be able to find another just like it.
You have 175mm crankarms now? Is it a large bike and do you have long legs?