I have never requested a buyer's phone number, but it is almost always part of the order details that Ebay supply to me.
I am not surprised that a business would send a marketing text - it seems a good way to increase their sales.
However, it does seem to break Ebay rules. If you wish, there is a link on this page to report the seller.
https://www.ebay.co.uk/help/policies/payment-policies/offers-buy-sell-outside-ebay-policy?id=4272
If you don't want them to contact you again, you can always block their number.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)