You know what's likely to annoy those customers of your competition and thus lower the chances that they will start purchasing from you, a seller who is an unknown quantity to them? Yep, that's right, see above.
You know what's a violation of eBay's member-to-member contact policy? Yep, you guessed it!
Feedback harvesting to spam buyers: it's not just annoying and against the rules - it is just plain lousy business practice. You're new and the first thing people learn about you is that you are either not very smart or you have no ethics. (or maybe both.) Potential buyers learn this; other sellers learn this; pretty soon you've shot yourself in the foot in your wild scramble to get a leg up by riding somebody else's coattails.
(Though my life would be a lot simpler if all of my intended competition were so bad at launching their businesses! Don';t get me wrong - I'm not a "how dare you sell supplies to my customers" person - first off, I don't own anybody and my customers are free to do as they please. Second, I don't make everything that people need; they couldn't only shop with me if they wanted to. Third, if I did make everything for everybody I'd have no time to ship it and sell it - there are only so many hours in a day, and there's only so much a business can grow when there's only one person doing the mixing and packaging and making and magic. I'd be miserable if I was the only person y'all shopped with. Hell, *I* shop with other suppliers - I've posted some about that before. That "my customer! stay away!" model is ridiculous and I don't hold to it. But I get kind of attached to my customers, and I even get protective of some of them sometimes when I work with them for a long time, and I don't like it when other so-called hoodoo practitioners or spiritual supplies vendors bug my people. And I mean "my people" in a very Alabama way, not in a business model way.)
Some people just don't think, and start trying to bust doors down before they really understand what they are doing. For the love of God, don't be "that guy." Everybody talks about that guy and nobody likes him.