I can't tell you for sure, but I would think that you still be perfectly understood. Unless there's a word pronounced just like selger but with the g, it shouldn't be a problem (and even then context and the fact that Oslo pronounces it that way would probably be enough to figure it out)
I would image that it's a bit like rhotic versus non-rhotic dialects of English. In some dialects (like General American or Canadian) all r's are pronounced, so that "hard" is pronounced /ˈhɑːrd/. Then there are the non-rhotic dialects (like most British English dialects or a Boston accent) which drop some of the r's, so that "hard" is pronounced /ˈhɑːd/. Most Americans can still understand English people and vice versa, so I bet it's the same way for Norwegian! :>
I don't know that I'd ever want to title an ad this way, but that's mostly because it's too imprecise - it doesn't have anything to do with the verb tense.
You're more likely to see something like "Kattunger til salgs" ("kittens for sale") as the title, but then you may find "Vi selger fire kattunger" ("We are selling four cats") inside the ad with some more specific info about them.