How much of a difference does developing a domain do for the value of a domain compared to just parking it?
So the question was “the value of a domain”. Let’s see. When one looking for some domain name for its project and you show him (her) parking statistic, it’s very good, it shows that people interested in this name, they typing it into the browser… Though, when you show the potential buyer developed site with the good number of unique daily visitors it’s better. Of course.
BUT!!!
1) It’s true ONLY for DEVELOPED site. You should hardly understand that to make website and to develop it (provide good position in search engines results and good quantity of visitors) – are completely different things.
So if you are able to bring your website into the top10 in Yahoo and Google and make good traffic (say several thousand of visitors per day) – forget about parking, simply develop your sites

. However, developed site is like the chicken, which lays golden eggs. Selling such a site is not too clever…
Well let’s back to the domain names.
2) Even a skilled SEO specialist need good time to develop 1 website. Now imagine, how long it will take to develop domain portfolio in 100 domains… Just to compare: parking of 100 domains will take several minutes, and couple of days for tuning…
3) It’s disputable, but actually, if your site is not well developed it could even decrease the value in some cases. Because when the buyer looking for the site (not for just name), he (she) first see at well developed sites.
Selling domain names and selling websites – little bit different things…
So, now you can decide what will be better for you in each particular case. Good Luck.