Hi Jaroslaw. My two cents...for what it's worth ~ it's good to have your work featured in groups. As far as sales, if the administrator and group members are motivated and actively bringing buyers to the group, as well as sharing the group link on their SM platforms, then sales will follow. Members also promote one another via likes, favs, comments, tweets, Instagram, Pinterest, etc., so groups can be very helpful.
As an administrator, I share both, features & our group link, on SM weekly. I also post homepage features several times daily, and give featured images at least 3 days, sometimes longer, to be seen & appreciated. It also helps to be in the 'Highly Active' top featured group rankings onsite. Visibility is key.
I can't imagine shoppers scrolling thru pages & pages of 'Images', so getting featured, and for more than one day, is best for exposure.
Contests are another way to get your work seen & sold. I've hosted nearly 3,000 since joining...and also routinely promote the winners on SM.
If my group 'A Woman's Touch' was not women-only, I'd ask you to join.
Best,
Brooks