Don't buy - date your guinea pigs at one of our recommended rescues for acceptance and character compatibility. All our recommended rescues have a mandatory quarantine and pregnancy watch. Most offer initial introductions/acceptance under expert supervision at the rescue as well as support during the settling in period and throughout the whole life of their piggies, so you get a LOT of service and neatly avoid all the pitfalls (clashing characters that don't work out, ill or pregnant piggies) that await the unwary owners. it is well worth to travel that bit further for the absence of major headaches and unplanned steep and often expensive learning curves!
These are the rescues we can guarantee for: https://www.theguineapigforum.co.uk/pages/guinea-pig-rescue-locator/
PS: Boars work best in pairs. Trios and small groups up to 10 piggies are particularly unstable. Baby boar trios have a fall-out rate of 90% during their first year and quartets are even worse.
If you have got sows, it is advisable to either date for a suitable neutered boar that will be accepted by the girls or rehome another bonded sow pair that is a different age to keep the dominance as clear and easy as possible, but neatly avoid any outsider problems.