What was the game?

Posts like this are very ambiguous without more details.
From what I can figure out with the sketchy info, it could be:

- Local people list a Star Trek Next Gen for sale $3500
---- they've seen them for sale at $7500 (might have), but theirs is broken
- Op feels he can find STTNG for 3K - 4K - and offers them $1500 (since it needs a $300 repair)
- Op feels they should have jumped at the deal - and comes to pinside for support

Sorry if this seems jaded - but I've witnessed pretty much this exact scenario - which seems pretty ridiculous.
Here's another jaded viewpoint - same scenario, but the game sells for $1500 - then the buyer brags about how he stole it and fixed it for $300.

But I could be totally off - just depends on what game we are actually talking about - which no one knows but the op.

Like others have said, if your offer is reasonable the best thing to do is make sure they have your name and number and be very nice (so they want to deal with you).

As to your original question...
- open pinside and show them the estimated value and show them others for sale and/or prices they've sold for
- then compare and adjust for the repair

That way your not just "telling them" with no authority other than a strangers word. They can think about it, do research themselves on pinside and will probably call you back if your offer is fair.