Yeah that would be nice, I can't see anyone buying many of my photos but it would be nice to be able to offer it without too much extra hassle.
The only other things I can think of is to make it a bit easier to see the photos of the groups your in, it only shows one group at a time on your 'homepage' and it doesn't always grab your attention so you don't go straight to it and start looking at them (or at least I don't). The other one is to have some sort of blog type functionality in it, rather than just subscribing to people's photos if there was a blog feature you could ask other people who are following you stuff or just put general stuff you've been up to on there.
For example there's a load of people who I subscribe to who do lots of sunrise photography using filters, I wouldn't mind getting into it a bit more but if I wanted some general hints & tips from them rather than messaging all of them asking I could just post up a new blog post and hopefully they'd answer. Would have been handy when I woke up at 6.30 this morning and decided to go to the coast to get some sunrise photos and well they don't look up to much at the mo