Hi Miles,
They will work well on their sides - the
S45 badge at the base of the frets swivels so it reads either vertically or horizontally so they are designed to work either way. They won't sound quite as good as they do on their stands slightly away from the rear walls but they are still very good.
The P45s referred to on this thread are very similar and are specifically designed to work attached to the rear walls - either vertically or horizontally. I have a pair and they do work extremely well with the BeoCenter 7xxx series, but from your description of where they will be used, the dimensions are slightly too large.
The real problem with buying S45s, particularly on eBay, is the possibility of damage in transit. S45s, and P45s for that matter, are both very prone to dislodging the magnets in the bass drivers in transit. Unless the magnets have been glued in place they are very prone to being dislodged and they sound awful if that has happened. It can be fixed and there are a number of threads referring to how to do that but if, like me you are a technical incompetent, it is a job best left to an enthusiast who knows what they are doing.
If you do risk a purchase on eBay, then ideally pick them up (after listening!) and cosset them during the trip home.
S45s will usually benefit from new capacitors in the crossovers, but these are available from Martin Olsen ("Dillen" on this site)
Speaking of Martin, he very recently sold a pair (in rare Oak finish) on eBay for less than the asking price of the pair you are looking at. These would have been virtually mint, fully working with good capacitors and with the magnets sorted for transport.
This probably won't help you make a decision on the pair you are looking at. I bought mine for a lot less - with their stands - but I had a round trip of 300 miles to get them, I had to restore the veneer and buy and arrange for someone to install a set of Martin's capacitors.
I think I would want them to be fully sorted for the sort of asking price and want them to be cosmetically pretty well mint.
Cleve