I wonder just how much money you think that is. Looking on Gunbroker, you could potentially pay anywhere from a few hundred dollars for a well-used shooter, to a couple of thousand for a NIB unfired specimen.
That said, if you've never used GB before I'd say the best advice I can give is,
don't be in a hurry. Figure out what kind of Model 19 you want - 2-1/2", 4", 6" - red ramp or patridge target sight - safe queen with box and all the goodies or beater or something in between - nickel or blue - etc. Use the Advanced Search option to narrow down the results. Find several auctions for guns that are the configuration that you want and watch them (without bidding) until they end.
You can also use the Completed Items tab to view auctions that have ended, although only the ones that have actually ended in a sale are useful. This will give you an idea of what guns like you're interested in are going for.
Once you've decided on what you're willing to pay for what you want, then you can start bidding. Like everyone else said, look at the seller's feedback. Always check and see how much shipping is, sometimes it can be significant. Don't bid more than your limit, if the bidding goes over another gun will come along. Like I said, don't be in a a hurry - if you haven't found what you want at the price you want in a couple of months, you might consider raising your bid limit. Or not, you never know when that bargain might come along (usually just
after you've already made a buy.)
I've been buying on BG for years and have made some pretty good deals, just by being patient enough to not jump on the first gun I see that meets my criteria. Fortunately you are looking for a gun that is relatively common so there will be lots for sale.
Good luck, and when you get that M-19 post some pictures here and tell us about how it went.