US Published Works
In the US, as opposed to the EU (since the question does not state a jurisdiction), if the artwork was published prior to the effective date of the US 1976 Copyright act (which became effective 1 Jan 1978 I believe), then it got copyright protection for 95 years from the date of publication, even if it was a work from a country which then had or now has a shorter term. This means that works published after 1924 and before 1978 may well still be in copyright.
Loss of Protection
Before the 1976 act, a work could lose its US protection if it was published without a copyright notice. A work could also lose its US protection if the copyright was not renewed. The 1976 act made renewal automatic for all works that had not yet reached their dates of renewal, so only works published before 1964 could lose protection in this way.
However, under the Uruguay Round Agreements Act (URAA), if a work was a non-US-Work, meaning that the author was not from the uS, or the work was first published outside the US and not published in the US until more than 30 days later, then any protection lost due to lack of notice or lack of renewal was restored if, on 1 jan 1996, it was still under copyright in its "source country". If copyright was restored, it got the full term that it would have had had it never been lost, which in most case would be 95 years from publication. The US Copyright Office publication Copyright Restoration Under the URAA describes this in detail.
Now all of the above applies only to published works. It is fairly easy to tell when a book, say, is published. But when is a work of art, say a painting, published? Some courts have said that simply being included in a public exhibit, even in a famous gallery or museum, is not publication. If prints or other copies are offered to the public for sale, or if a copy is included in a published book, that is clearly publication.
The question asks:
Is there a possibility that someone else has bought the right to the artwork and therefore owns the right to it even after 70 years have passed from the original authors death?
When a person buys the copyright in a work, they only buy the rights that the previous copyright owner (often the creator or the creator's heir) had. Such a sale does not length the period of protection.
However, if the work was prepared by an employee as part of his or her employment, it is a "work-made for hire". In that case, under US law (but not I think in the UK) the employer is considered the "author" and the employee's detah dater is irrelevant. If the employer is a business, US copyright now lasts for 120 years after creation, or 95 years after publication, whichever is shorter. That could be longer than the life+70 years term, but would rarely apply to artwork.
If one is interested in copyright protection under US law it is not sufficient to determine the date of the author's (artist's) death. One must find out whether the work was published, and if so when.
While all copyright law in Berne Convention countries has many basic similarities, the convention leaves it to individual countries to set the length of copyright, provided at least a minimum term is granted