Using vectors/stock graphics in designs?

I'm looking for a little advice, hopefully somebody can offer me some.

I currently work in Marketing/Graphic Design.I do not have a degree in Graphic Design, but a degree in Advertising (which was very design heavy) so I came out of university with good design skills. In my current job, I do very basic things such as mail shots, website graphics, leaflets, brochures etc. where the use of stock photography and imagery is fine a) because of time-constraints and b) because we obviously do not sell anything.

I would love to start designing things like invites, announcements and wedding stationery. As many of my friends and colleagues compliment me on my eye for design, good use of space and images etc when I design for them. Of course, when I design for myself or friends, I use whatever comes to hand. Sometimes I incorporate free vector packs (of things like intricate swirls/flowers or silhouettes of birthday cakes or some baby vectors to enhance a baby shower card) and sometimes I even buy some off places like shutterstock. A friend wanted a birthday card for his girlfriend which had a typewriter on (as she is a writer) and i made a lovely card using a drawing of a vintage typewriter as the focus with good use of typography.

Without using these resources, I am afraid that my skills are very limited. I am very good with colour, layout and typography. And very good with coming up with wording and beautiful unique ideas. But I cannot draw to save my life, and I definitely cannot make vectors of my own (at least not brilliant ones - i do have some limited knowledge of using illustrator)

My question is, am I unsuited to selling my designs online as I rely too much on stock images to help pull designs together? I absolutely love doing it, but if I took out graphics that I didn't design myself, I fear I would only be able to take advantage of typography and colour. Do I absolutely need to learn to draw? Would I be better off learning these skills before even trying to design invitations to sell? Or is some use of stock graphics okay? I am not entirely sure what is acceptable in the industry. Sometimes I see shops on Etsy selling cards with what looks like stock graphics/vectors and wonder if they are the original artists or not.

Thanks for any help/advice you might have. Very curious to hear your answers.