I looked up the word sêls in the Ap Geiriaduron, but sêl means either 'seal' or 'ardour, zeal', and 'sale' translates as gwerthiant.
So I presume this is a modern plural-only word borrowed from English, only used in shops or stores for advertising goods on sale at reduced prices, while gwerthiant would be used for 'company sales', 'sales manager', etc.
Is this correct?
sêl (sêls) is also used for 'a sale (sales)' and is widely used in colloquial Welsh, especially for a sale at reduced prices. As you say, it is a loan-word.
For 'sales manager', The Porth Termau Cenedlaethol Cymru (look for it on-line) gives rheolwr gwerthu. This will be on the basis that it is a role managing a continuous activity - there would not be much much point in employing 'a manager of a sale'!
If I have understood the meaning of the word, for instance, 'car boot sale' would be sêl cist car (provided in Welsh the literal translation of the English expression is used).
'Gwerthiant cist car' is also in some areas where people prefer to use more authentic Welsh words, probably in one of the 'Papur Bro' community newspapers.
So the two words sêl and gwerthiant do overlap in some degree.
Thanks for clarifying!
The person and number come from i..., and that is followed by the soft mutated verb-noun, not by a conjugated verb. The tense comes from other context. There are many i-dot patterns like this in Welsh.