The golden rule: Do not offer free samples. If you’re a serious seller, you have previous work*. Show it to them. If they agree, great, if they don’t, move on. If your work consists of something you can provide samples without having to do the whole work in advance, offer them a sample for only $5. Especially if it’s a big project, e.g. $1000, your prospective buyer will accept it without any further discussion. And from my point of view, it’s completely understandable. If I buy something B2B, I don’t care about five lousy bucks. In the time, I’m discussing with the provider, I could earn twice or multiple times the amount.
I’m sometimes a bit confused about the effort, these scammers perform for a $5 service.
*Make sure that you have your buyer’s permission to use their work for promotional purposes.