Re: Too Hard?
Sorry, but it's time to get with the times.
Companies like iZettle will sell you a black-box solution for a hundred-or-so pounds. You can get a smartphone-connected thing that will set you back £70 but relies on the security/compatibility/availability of a smartphone. The rates are quite low, the gadget is one-off, it's Chip&PIN-capable (and also magstripe if you deal with foreigners) and can tie into accounting apps for you.
Last time I went to an antiques / bric-a-brac sale, most stalls took cards using things like iZettle and clearly advertised so. Sure, you may have needed to spend £10 to make it worth their while, but rather £9 from a transaction than nothing because you didn't have the cash. If your profit margins aren't already taking account of such things then you aren't going to be in business long, mom'n'pop or not.
Cheque fraud went through the roof in recent years - cheques nearly aren't accepted by BANKS any more, and the fallback when they bounce costs the retailer a lot more - why do you think they charge for that? Cash is expensive and risky to handle. It's much more likely those using those methods are actually able to not declare that income, in fact! Card payments? A cheap one-off purchase of a black box from a website, swipe the card and it pops into your bank account - with accountability, complete records, instantly revoked if stolen, you know they have the funds available, and a charge of fraud if it's misused by the owner.
Hell, I bought an iZettle for my girlfriend for Christmas so she can sell her pottery. She fires up the kiln maybe twice a year. Mom'n'pops have no excuse and shouldn't be handling cash or cheques nowadays.