Is money earned from collecting customer orders and purchasing goods after advance payment considered unlawful money, and is the interpretation of a dream about spitting blood, which indicates earning unlawful money, correct in this case?

Interpretation of dreams should not rely solely on books, and it is a sin for an ignorant person to attempt it. As for your work, if you take orders from customers to procure their goods with their knowledge, this falls under the category of brokerage (ja'ala and samsara). In this case, the fee must be clearly specified, and what is taken from the customer should only be the cost of procuring the permissible item. However, if you take orders and money as a sale when you do not possess the item, this is impermissible due to the Prophet's ﷺ prohibition against selling what one does not own. An alternative solution is to use the salam (advance payment) system, whereby you take the money and agree with them on the goods that will be delivered in exchange for the advance payment.