@Jen
What date was the advert? Are these recent issues?
Asking because I've realised from looking at #scientific #journals in this period that periodicals for the international trade often travelled as bound volumes (not issues; thus, not immediately); and that those volumes also circulated through the secondhand trade (sometimes decades later). But I'm not sure if back issues of the Lady's Mag were in demand in the same way as back issues of Philosophical Transactions...
My examples come from Philosophical Transactions (a v diff sort of periodical...)
1) In the 18thC, there weren't reprints, but there were 'abridgements'. By squeezing 21 volumes of back run into just 3 vols in 1705-8, entrepreneurial third-parties made the contents vastly more available. There may have been twice as many copies of the abridgements in circulation as the originals. More abridgements followed, to 1809 (see pp.133ff of our book https://www.uclpress.co.uk/products/187262 )
#bookhistodon
3) But, to get to the point about reprints: in the 20thC, there were full-reprints of the old Phil Trans. Photographic methods of generating litho plates (i.e. avoiding the need to reset all the text) made it possible to reprint the 17th and 18thC volumes (and even the 19thC vols) in the 1940s-70s. Also, microfiche and microcard editions. (e.g. p.486 of our book)