Instead of pointing it out perhaps ask the seller "hey, that upper radiator support has been welded, do you know anything about it?"
I used to send PM's to sellers vs asking a question like that on chat, but I am finding there are two types (speaking in broad terms), the sellers that are super transparent and the sellers that either are not or are clueless.
In either case that second bucket of sellers has the ability to pull the wool over unsuspecting buyers, so now - mostly, not always - I just ask in the chat/comments.
In this case I would be glad to ask the question, but I could not back up any kind of retort that may be misleading as I don't have your knowledge on this issue (or frankly many others when it comes to these cars).
I'm still reeling from the time I couldn't get the free dishwasher from the ebay guy in R.I. selling that MB