The best thing for you to do, is to ask her if she’s single, and then ask her if she’d like to go out with you! It’s that simple. 🙂
Of course, use your own style. Rather than just blurt out the questions, you can call her and tell her how nice it was having her sell you your apartment. She’ll thank you, profusely, and then you can tell her that it was specifically nice that she was the one doing the selling, and if you happened to be lucky enough to find out that she was single and available, you’d love to invite her to dinner. That leaves the ball in her court, and she’ll either tell you then that she’s married, in a relationship or (DING! DING! DING!) single and available and would love to have dinner with you.
If that’s the case, tell her, great, and make a date for the following week or weekend.
That’s just a model you can use, but basically, all you have to do is get her phone number, call her, and find out if she’s available.
Hope that helps! 😀