I can understand why to an extent Sedo implement a block, however it is unfortunately flawed. Like you say, a search every 15 of so seconds is certainly distinguishable from a search every second, or multiple searches per second.
Perhaps a more sensible block would be based on the number of searches in a given timeframe, whether or not you are logged in (i.e. an actual Sedo user), the number of domains you currently have listed on Sedo and whether or not you are an active buyer.
The biggest issue I found with the block is not only does it not tell you that you are blocked, the block applies to the entire Sedo site (they may have fixed this by now), which is a bit of an issue when you are actually in the process of either selling a domain name or are in the process of attempting to buy one.
It certainly needs a rethink imho.
As for Sedo not being able to generate a list for you, that's not based on some technical restriction as it would be quite easy for them to run a search against their database using your keywords. If they genuinally don't have a tool to allow them to do that, then it wouldn't be partically time consuming or difficult for them to create one so that they could offer to do it for genuine buyers. At the end of the day they're there to sell domains and if a user like yourself is looking to aquire a number of names then they should certainly be receptive to that.
