Sorry to say, but you'll learn nothing while mining litecoin with an rPi. You defenately won't mine enough to buy anything (whatsoever) on any shop anywhere.
https://litecoin.info/index.php/Mining_hardware_comparison#CPUAn rpi4 (latest model) has a quadcore Cortex A72.
The A72 isn't in the above list, but it's clear that these CPU's seldomly hash at a hashrate higher than 5 kH/s
Enter this number into one of the many mining calculators like this one (first his on google)
https://www.cryptocompare.com/mining/calculator/ltc?HashingPower=5&HashingUnit=KH%2Fs&PowerConsumption=0&CostPerkWh=0&MiningPoolFee=0The ESTIMATED income, per YEAR is 0.00007916 Litecoin.
At current preev rate, that's less than half a dollarcent (in other words, you'll have to mine 200 years at current diff, block reward and exchange rate to mine $1)
Even if you find a pool that accepts really low diff shares, the chance you'll find a share is very low... So basically, mining litecoin with an rPi is just wasting cpu cycles while nothing at all happens.
As for who is accepting LTC, there are payment gateways like coinpayments that accept a lot of established altcoins (as long as the shopowner configures them). I'm not a fan of coinpayments, but you might look trough their business directory and check out their vendors to see if you can find one that accepts LTC
