Marketing exposure is an important part of selling your artwork but there's a lot of questions you need to ask yourself when invited to join anything...be it an online gallery, offer of publishing or even an art show.
The most important question is if it will cost you money. Some expenses for marketing are to be expected and worth the hit on your budget. But most solicitation has the sending party's interest in mind first and foremost.
Keep in mind that online gallery type places have expenses of their own...servers cost money. But ask yourself what are they doing to actually give you exposure to possible clientele. Just giving you a place to show your work is not enough, you can do that yourself with a little research on website building and search engine rankings.
Publishing is another matter...so far I've only seen come ons. I've had my work in artist magazines and the invites come from either the publishers or someone who wanted to use my artwork to help sell their product...I didn't get paid, but I got great exposure and it was free. Any artbook of great artwork is going to concentrate on name artists and the publishers absorb the cost, not the artist.
And art shows...this is just personal opinion but the majority seem to make the most money from entry fees. Think how many artists submit...the odds are slim that your art will place. When I decide to enter I only go for the name shows that truly are a benchmark accomplishment if you place. I really check out the judges to see what kind of work they lean towards and see if my work fits in that niche. I also research past winners to see what caliber of work makes the cut.
When you're a starving artist every penny counts. Be wary of something that offers the moon while reaching in your pocket.
Note...speaking of mailboxes, I got my Robert Genn Weekly letter today and it seems there's a new Golden acrylic line out there with an open drying time. I don't use acrylics often but having worked with oils for so long I can see the possibilities.