As a former business owner here I will try to reply to your questions....
Learning Castillian Spanish good.......understanding Canarian Spanish (closer to South American dialect as opposed to mainland ),,,can be difficult..they will understand you...not so easy to understand them
Unemployment still around 30% even though the island is full of tourists
Stock...depends what it is ..anything sourced outside the islands will attract customs duty (varies according to what it is you import ..but figure on 20%.)...plus the local Vat Known as IGIC which varies between 7% to 13% again dependant on what you import.Sourcing locally could be a problem as if it is not food/drink or a locally produced item(s) then you will struggle.
You cannot open a business without a licence from the local council...costs vary according to which licence you require...and as to setting up a stall on land...big no-no- ...you simply would not get permission
...even market trading requires proof of tax registration,proof of social security registration proof of residency and proof of empadronamiento .(.local council registration of abode)...before they allow you on a waiting list for the next available stall
Costs of going self-employed are some of the highest in the EU.....autonomo( self -employment) social security payments start at 260€ per month ...and go up (and up ..and up )...then add taxes at around 20%----and all this has to be paid whether you are earning or not...from day 1
Business here can be a bit ´´cut-throat´´´though fellow Brits/Irish are far worse than any local for undermining you ..especially if in competition locally
...make sure all your licences are in place and that everything is totally legal ( paperwork wise ) or you may come unstuck very quickly
There are in fact 7 or so inter-related families who have fingers in many ,many ,pies here...but they are not the problem unless you make it one
Sorry to be a bit vague ..but without info on what you intend to do or sell (AND do not post it on here...or by the time you get here there will already be established competition
)....keep your plans and ideas to yourself
OOps sorry missed out on business accounts....advisable to have one as if you miss a payment to any official body, they can and sometimes are ,´´embargoed´´..in effect ´´frozen´´ until you pay up...also costs of banking here....no such thing as a free bank account in Spain..and business accounts will be more expensive than personal ones
....like always... I try to end on a good note.....lol
Edited later ++ forgot to tell you that import anything you will need a Spanish Govt issued EORI number ...more costs and much more paperwork (they love that here !!) and also the yearly costs of an official accountant and for at least the first year a ´´gestor´´ (translator.paperwork person) until you get the hang of it.