You've said an author is tainted by poor sales and therefore unlikely to find an agent willing to represent them. But is this true across the board, with no consideration given to who the publisher is or whether an advance was given? There are so many small publishers out there and most offer little to no help in the way of marketing. I can see if an author signed with a big house and didn't earn back the advance. But what of an author who signed with a small boutique publisher and received no advance and little to no help with marketing? Are those kinds of circumstances considered at all, or is everyone thrown into the same you'll-never-find-an-agent pot, gooses cooked?
It's all so relative it's hard to make a blanket statement.
Sufficient unto the day to say, no matter the circumstances, it's easier to shop an author with no sales figures than to shop one with sales figures that need to be explained.
And it hasn't eluded me that "shopping" someone in the parlance of spy fiction means to inform on them.
When you sign with a "small, boutique publisher" you should know what you're getting into. Of course there isn't going to be marketing or publicity help. There are (most likely) not going to be books in stores. You're going to need the online retailers to reach your target audience, and you're going to have to do it yourself.
Expecting anything else is wearing rose-colored glasses and renaming yourself Pollyanna.
It's worse with digital publishing because I used to be able to say "he sold all the books they printed" but with ebooks, there is no limit to the number available for sale. And ebook sales are assessed the way mass market paperbacks used to be. Because of the lower price (compared to hardcovers) you have to sell MORE to get attention. 300 ebooks is a blip. 3000 ebooks is ok. 30,000 ebooks is more likely to get noticed.
This is your career, and you get to do what you want but you need to know before you sign with any publisher what it will mean long term.
Everyone thinks they are going to sell more than they do. EVERYONE. If you think you can sell 10,000 copies, you'll be astounded to find you only sell 100. I won't be surprised in the slightest. Getting people to buy things is a VERY tough job.
Selling is hard work. It's not just a matter of saying "hey, here I am, aren't you glad to see me?"
Think of it like Miss America. Those ladies walk down the runway in Atlantic City and they look effortlessly beautiful don't they? Tall, slim, fit, talented, poised (ok, mostly). Yet, it's not effortless in the least. Those ladies have spent YEARS honing their talent, practicing their walk, learning how to apply makeup, and entering local and regional pageants to practice. They've worked hard to make it look easy.
Book sales are exactly like that. You spend months gearing up, building your mailing list, making friends, learning terminology, figuring out where your buyers are. And then you spend months actually doing the marketing and publicity.
It looks easy when the other guy is doing it.It's never easy when you're doing it.
To answer your question: while it's certainly easier to explain low sales numbers if your publisher is Podunk, Puny and Smalls, LLC, what any future publisher sees is a book no one wanted to buy. Publishers are risk averse by default. Telling them not to be afraid of low sales numbers isn't as persuasive as saying there are no low sales numbers.
What does that mean for you: if you're published by a small press prepare to become a salesperson for your book.