It doesn't bother me if some people buy sets with plans to sell them in the future.
I'm just happy that someone is buying the set from Mosaic in the first place. I figure the more sets they sell, the better the chances that they'll stay around and be in business for a long time.
These days it's pretty easy to keep track of a Mosaic set's availability status. I guess I would only be bothered by this subject if a set just hit running low and one person came in and bought ALL the remaining sets. I know it's there right, but it wouldn't be cool IMO.
I've used the argument with my wife that Mosaic sets increase in value. That has really helped me convince her to buy them for me as gifts or look the other way as my collection has grown.
I've never sold any of my sets, so you can see I'm not just investing in them, but sometimes I see dollar signs in my wifes eyes as she has looked at my sets.